easysvm: a visual analysis approach for open-box support...

Computational Visual MediaDOI 10.1007/s41095-xxx-xxxx-x Vol. x, No. x, month year, xx–xx

Article Type (Research/Review)

EasySVM: a visual analysis approach for open-box support vectormachines

Yuxin Ma1, Wei Chen1�, Xiaohong Ma1, Jiayi Xu1, Xinxin Huang1, Ross Maciejewski2 and Anthony K.H. Tung3

c© The Author(s) 2015. This article is published with open access at Springerlink.com

Abstract Support vector machines (SVMs) are supervisedlearning models traditionally employed for classificationand regression analysis. In classification analysis, aset of training data is chosen, and each instance in thetraining data is assigned a categorical class. An SVMthen constructs a model based on a separating plane thatmaximize the margin between different classes. Despitebeing one of the most popular classification models becauseof its strong performance empirically, understanding theknowledge captured in an SVM remains difficult. SVMs aretypically applied in a black-box manner where the detailsof parameters tuning, training and even the final constructedmodel is hidden from the users. This is natural since thesedetails are often complex and difficult to understand withoutproper visualization tools. However, such an approachoften brings about various problems including trial-and-error tuning and suspicious users who are forced to trustthese models blindly.

The contribution of this paper is a visual analysisapproach for building SVMs in an open-box manner. Ourgoal is to improve an analyst’s understanding of the SVMmodeling process through a suite of visualization techniquesthat allow users to have full interactive visual control overthe entire SVM training process. Our visual explorationtools have been developed to enable intuitive parametertuning, training data manipulation, and rule extraction aspart of the SVM training process. To demonstrate theefficacy of our approach, we conduct a case study using areal world robot control dataset.

1 State Key Lab of CAD&CG, Zhejiang University,Hangzhou, 310058, China. E-mail: [email protected],[email protected], [email protected],[email protected], [email protected].

2 Arizona State University. E-mail: [email protected] National University of Singapore. E-mail:

[email protected] received: 2014-12-31; accepted: 2015-01-30.

Keywords Support vector machines, rule extraction,visual classification, high-dimensionalvisualization, visual analysis .

1 Introduction

A support vector machine (SVM) [14] is a supervisedlearning method that is widely used in a variety ofapplication areas, such as text analysis [41], computervision [36] and bioinformatics [20, 23]. The SVM model isa discriminative model which tries to split the training datainto the two classes by creating a separating hyper-plane atthe place where the two classes are furthest apart. The classof a new data point is predicted by determining which sideof the hyper-plane it laid on.

While SVMs have been shown to have high accuracy inclassification [1], they also face a variety of challenges whenwe want to use them for data analytics. First, conventionalSVM approaches are “black-box” schemes in which detailsof the model construction and prediction process are hiddenfrom the user. The user simply provides the SVM witha training dataset, relevant input parameters, and a modelwill be constructed for making prediction of unlabelled data.Other outputs that can be retrieved from the trained SVMmodel are a vector that represents the feature weights, and aset of training data instances called support vectors. Theseoutputs are unable to provide any insights for domain userswho want to better understand the reasoning behind certainclassification results. Thus, while the automatic “black-box” model releases the user from laborious interactions, itmay hinder the user in terms of understanding and insight.Previous work from Tzeng et al. [42] indicates that userscan gain much insight if he/she is allowed to apply function-based techniques that can be explained and validated. Suchfunction-based methods enable the interpretation of theclassification process with respect to the application domain.The importance of gaining such insight had motivated data

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mining algorithms [33, 35] that try to extract “if-then-structured” rules from the classification model.

Another issue that makes gaining insight from SVMsdifficult is the use of non-linear kernels [39] whichtypically improve the classification accuracy. There is,however, a lack of systematic and effective methods toselect the correct kernel functions and input parametersto give good classification result [30]. As such,tuning parameters in this black-box environment can beextremely difficult and time-consuming for the user. Inaddition, the non-linear characteristic further complicatesthe difficulties of interpreting the classification process.While recently developed local model techniques [21,30] have effectively reduced the complexity of non-linearSVMs by approximating the boundaries with multiple locallinear planes, interpreting the complex patterns and datadistributions at the boundaries remain complicated.

In order to overcome these challenges, we have designedan open-box approach where the user is embedded intothe modeling process and equipped with tools to exploreand study complex circumstances. We believe the key toshifting from a “black-box” to an “open-box” approach isto empower the user with a visual analytics interface whichwill enable a better understanding of SVMs, the underlyingdataset, and the classification process. Specifically, ourinterface design focuses on these three questions:• Q1: How can we enable the user to be better informed

of the training dataset and model building process ofSVMs?• Q2: How can we enable the user to effectively

understand non-linear decision boundaries and buildlocal models that fit the boundaries?• Q3: How can we enable the user in interpreting and

manipulating the prediction results in a user-friendlyway?

This paper presents our efforts in opening the black-boxof model building and knowledge extraction from SVMs.In this paper we propose our design and implementationof a web-based visual analysis system that supports modelbuilding using SVMs. To the best of our knowledge, thispaper is the first to address the issues of open-box analysis ofSVMs in visual analytics literature. The main contributionsinclude:• An interactive visualization method for exploring data

instances, linear SVMs and their relationships;• A visual analysis approach for building local linear

SVM models that overcomes the non-linearity of theunderlying dataset, and;• A visual rule extraction method that allows the user to

extract rules that best interpret the models.The remaining sections are organized as follows. Related

work is covered in Section 2. Section 3 presents anintroduction to SVMs. The next section describes ourvisual analysis solutions for three tasks: model buildingand explanation, local SVM building and rule extraction.Section 5 demonstrates the effectiveness of our solutionthrough a case study on robot control, followed bydiscussion in Section 6 and conclusions in Section 7.

2 Related Work

The work presented in this paper is related to three broadtopics: 1) support vector machines; 2) visual exploration ofhigh-dimensional data, and; 3) visual classification.

2.1 Support Vector Machines

The SVM is currently regarded as state-of-the art inclassification techniques [1], and studies have revealed thatSVMs perform well when compared to other classificationtechniques [5]. This performance can be partly attributedto the use of non-linear kernels which unfortunately make itdifficult to interpret the models. In addition, the productionof the boundary function is often quite difficult. Work byWahba et al. [44] explored the use of SVM functionalsto produce classification boundaries, exploring tradeoffsbetween the size of the SVM functional and the smoothingparameters.

In practice, it is very difficult to tune an SVM for non-experienced users [26]. However, recently, a set of methodswere developed to simplify the model complexity whilemanaging non-linearity [18, 32]. One representative schemeis to build multiple local models to approximate the globalone [7, 30, 45]. Local modeling methods train a seriesof linear SVM models near decision boundaries by usingtraining data instances in the corresponding local regions.Unlabelled instances are then classified by the nearest localmodels in the prediction stage. This method is able toapproximate the non-linear boundaries by a series of locallinear ones. No intuitive indications are yet given by thelocal models about how complex the local regions are, orwhat kind of patterns are presented in the regions.

Rule extraction is an important component for theinterpretation of SVMs or other classification techniques [6,19]. Martens et al. [33] provided a comprehensive study onrule extraction of SVMs. These methods commonly employan automatic optimization process and result in an axis-parallel representation. However, the targets and interestsmay vary according to the user and analysis tasks. It islikely that a visual analysis process can enable the user toexplore both the input parameter space and the classificationboundaries.

Unfortunately, there is little work that is dedicated tovisualizing SVMs. Caragea et al. [9] applied a projection

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EasySVM: a visual analysis approach for open-box support vector machines 3

method to transform data instances onto 2-D space. Theseparating hyper-plane is sampled in the data space, andprojected on to the 2-D plane. The work by Aragon etal. [2] utilized SVMs as part of a visual analysis systemfor astrophysics, but provided no general support for SVMexploration. In [4], we presented an interactive visualizationsystem for visualizing SVMs and providing interactions forusers to perform exploration in the dataset.

2.2 Visual Exploration of High-Dimensional Data

One key challenge in opening up SVMs is the need forhigh-dimensional data exploration methods. Recent workin this area has utilized multi-dimensional projections tomap data instances in high-dimensional data space to thelow-dimensional (2-D) space. The key issue is how toexplore the underlying dataset with informative projections.Previous works, such as grand tour [3] and projectionpursuit [17], generate a series of projections that allowthe user to dynamically explore various lower dimensionalprojections of the data in a systematic way in order tofind a preferred projection. Other exploration techniquesfacilitate the user with controls of the projection matrix (e.g.,[8, 13]). Nam et al. [34] proposed a projection navigationinterface, where the exploration and navigation activitiesare decomposed into five major tasks: sight identification,tour path planning, touring, looking around & zoominto detail, and orientation & localization. Additionally,an N-D touchpad polygon is provided to navigate inhigh-dimensional space by adjusting the combination ofprojection weights on each dimension.

Alternatively, high-dimensional data can be visualizedwith scatterplot matrix [12], parallel coordinates plot(PCP) [27, 28] and radar charts [10]. Previous workhas also employed interactive exploration and navigationamong scatterplots to fill the gap between projections andaxes-based visualization techniques. Elmqvist et al. [15]presented an interactive method to support visualization andexploration of relations among different 2-D scatterplots inhigh-dimensional space. Similarly, 3D navigation [38] onthe basis of rigid body rotation can be employed for viewing3D scatterplot matrices. The 3-D rotation interactionimproves the user’s ability to perceive corresponding pointsin different scatterplots for comparisons.

2.3 Visual Classification

Some visual classification approaches, such as decisionand rule-based classifiers [31, 37], employ so-called “white-box models”, with which the detailed process is easy tounderstand. Teoh et al. [40] considered the process ofbuilding decision trees as a knowledge discovery method,and argued that visualization of the decision tree model

can reveal valuable information in the data. Elzen etal. [43] presented a system for interactive constructionand analysis of desicion trees with operations includinggrowing, pruning, optimization and analysis.

Another category of works focuses on designing model-transparent frameworks where the user is allowed to providetraining datasets and view the results. Thus, the low-level classification techniques can be directly employedwithout modification of the analytical process. Heimerl etal. [24] proposed to make a tight integration of the user intothe labelling process and suggested an interactive binaryclassifier training approach for text analysis. Hoferlin etal. [25] presented a system to build cascades of linearclassifiers for image classification.

For open-box visual analysis approaches, one of the mostsimilar works to our approach is from Tzeng et al. [42]. Itcombines several visualization designs for artificial neuralnetworks to open the black box of underlying dependenciesbetween the input and output data. Unlike our interactivevisual analysis approach, their open-box scheme is limitedto present a static visualization of the model structureand does not provide means of data exploration andinterpretation of classification process.

3 An Introduction to SVM Classification

Given a set of training data points each with m attributesand an associated class labels, the SVM attempts to separatethese points by a (m− 1)-dimensional hyperplane. In thissection, we will briefly describe this process with the helpof Figure 1.

K(xi, xj) = Φ(xi)Φ(xj)

wTx + b = -1

wTx + b = 0

wTx+ b = +1

w

ξ =0

ξ < 1ξ > 1

b

Functional Margin =

MisclassifiedPoint

Support Vector

Support Vector

2√wTw

Fig. 1 An illustration of a linear support vector machine.

Suppose that xxxi ∈ Rm, i = 1,2, . . . ,n are n training datainstances in two different classes, and yi ∈ {−1,+1}, i =1,2, . . . ,n are their corresponding class labels. A linear

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support vector machine aims to construct a hyper-planewwwT xxx+b = 0 (1)

in the m-dimensional data space Rm that has the largestdistance to the nearest training data instances of each class(which is called “a functional margin”).

Equation 1 can be solved by computing the followingoptimization problem:

minwww,b,ξ

12

wwwT www+Cn

∑i=1

ξi (2)

subject to yi(wwwT xxxi +b)≥ 1−ξi,ξi ≥ 0, i = 1, . . . ,n.

where C is a user-adjustable parameter to control the relativeimportance of maximizing the margin or satisfying theconstraint of partitioning each training data instances intothe correct half-space. The dual problem of Equation 2 isderived by introducing Lagrange multipliers αi:

max12

n

∑i, j=1

αiα jyiy jK(xxxi,xxx j)−n

∑i=1

αi (3)

subject ton

∑i=1

αiyi = 0,0≤ αi ≤C, i = 1, . . . ,n.

Here, K(xxxi,xxx j) = Φ(xxxi)Φ(xxx j) = 〈Φ(xxxi)Φ(xxx j)〉 is called thekernel function. For a linear SVM, its kernel is the dotproduct of xxxi and xxx j, i.e., K(xxxi,xxx j) = xxxT

i xxx j. Support vectorsare those training data instances xxxs whose correspondingLagrange multipliers is above zero. Finally, the decisionfunction for classifying a new data instance xxx is:

y = sgn(n

∑i=1

yiαiK(xxxi, xxx)+b). (4)

4 EasySVM: Open-box Visual Modeling ofSVMs

As previously stated, the shortcomings of SVMs lieprimarily in the fact that they are difficult to interpretand explore. A general visual modeling approach for alinear SVM consists of two components: 1) visualizationof the training data instances, the SVM model andinteraction between data instances and model; 2) user-guided construction of the SVM model. Both componentsare basic operations of the entire analysis process. Figure 2shows the analytical scheme for our solution which consistof three components:

Open-box Visual Modeling for SVMs To enable theuser to quickly determine item relationships, we map thedata instances and SVM models into a 2-D plane with anorthogonal projection. We design an interactive projectioncontrol scheme to support the flexible exploration of thedataset and the model from different aspects.

Local SVM Building through Visualization Oncethe data instances and models are visualized, the usermay recognize non-linearities within the model space.

The underlying SVM model can then be progressivelyapproximated with a sequence of linear localized SVMmodels. A suite of visual linkage and comparison operationsare integrated to enable analysts to explore relations betweendata instances and SVM models as well as manipulate localmodels.

Visual Rule Extraction Rule extraction is interactivelyperformed along each axis. The user can either selectsegments on the axes or select regions from the projectedresults of data instances and the SVM models. Eachextracted rule can be represented with a hierarchical treestructure.

This scheme is encapsulated in our EasySVM system,a web-based interactive visualization system depicted inFigure 3. The system consists of four main views:a scatterplot view, a projection management view, adimension selection view, and a rule extraction view.

Open-box Visual Modelingof Linear SVM

Visual Local SVMBuilding

Visual Rule Extraction

Training Data Instances

Classification Rules

Analytical Loop

Fig. 2 Overview of our approach.

4.1 Open-box Visual Modeling of Linear SVM

The traditional SVM model building process can besummarized into following fours steps: 1) preprocess thedata, 2) select parameters, 3) train the model, and 4) validatethe training result [26]. If the validation result cannot passthe test (e.g. low prediction accuracy on the test dataset), theprocess will restart from step 2 again until it meets the user’srequirement. In our visual model building process, eachof the model building steps aforementioned is enhancedby interactive visualization and visual exploration methodsto facilitate understanding of the model building process.Meanwhile, additional data exploration and manipulationcan be performed at any model building step. Figure 4 (a)shows our iterative visual model building process:

Data Exploration and Initial Training The user canexplore the training dataset to approximate the classificationboundaries. Then a global model is trained with all trainingdata instances and an initial parameter C in the global SVMmodel building panel (Figure 3(c)). After this initial step,the user can perform analysis and operations iteratively inthe following two steps.

Visual Model Exploration and Validation For a trained

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Fig. 3 Interface of EasySVM: (a) the data view, (b) the view of multiple projections; (c) the SVM model building view, (d) the rule extraction view, and (e) thedimension selection view.

model, an initial projection is generated at the direction ofthe side view. The user can evaluate the model startingfrom the side view to locate misclassified instances, checktheir distributions and patterns with projections, view theboundaries near the separating hyper-plane, and makedecisions on data operations. Compared with the traditionalmachine learning procedure, the visual exploration of thetraining result is devoted to providing insight of the reasonwhy the training result is. Meanwhile, the predictionaccuracy on its own training dataset is computed anddisplayed as another reference for model validation.

Data Manipulation and Parameter Tuning Afterexploration, some training data instances that affect themodel building can be modified by changing labels ordeleted from the dataset if they are considered to be noiseor instances with invalid values. In addition, the parameterC can be tuned in this step to balance the trade-off between

margins of the hyper-plane and prediction accuracy on itstraining dataset. It is required to re-train the model afterthese two operations to update the model and classificationresults. The model building process stops when thevalidation result reaches the user’s requirement, such asprediction accuracy on a test dataset. It should be noted thatfor a dataset with non-linear or even more complex decisionboundaries, local linear models are needed.

4.1.1 Visualization of Training Data and the SVMModel

The data view (Figure 3(a)) in our system is basedon a scatterplot in which data instances are projectedinto a low-dimensional subspace. We use an orthogonalprojection to embed high-dimensional data instances andthe SVM model in a 2-D plane. This view features twopanels: a top menubar that contains exploration tools forthe scatterplot, and a projection control panel that provides

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Visual Model Exploration & Validation Data Manipulation & Parameter Tuning

Training

Identification of ROIs

Selection of Training Data Visual Validation

Training

(a) Global Model Building

(b) Local Model Building

Data Exploration & Initial Training

Non-linear boundaries found

C=10

Possible outliers, noises, etc.

Instancesmanipulated

C=0.1

Visualization ofmultiple local models

Potential non-linear boundary

ROI

Local SVM #1

Local SVM #2

Local SVM #3

Fig. 4 (a) Global SVM model building process. If a non-linear decision boundary is found, the user can enter the local model building process illustrated in (b).

visualization of dimension axes and control methods ofinteractive projection control. Two reasons for that are: 1) itis a powerful technique for visualizing training dataset, and2) it makes the geometrical relations between data instancesand the hyperplane be held at the same time, like relativepositions and distances between each other that are essentialfor the user to understand the structure of SVM models.

Orthogonal Projection Given an orthogonal projectionmatrix AAAm×2 = [ fff 1, fff 2], two m-d vectors fff 1, fff 2 span a 2-D plane in the m-dimensional data space where all datainstances involved are projected. Applying it to a high-dimensional data instance yields a corresponding data pointon the 2-D plane, i.e. the coordinates of data point in thescatterplot view xxx′i = xxxiAAA. It should be noted that the 2-D projection formula of the separating hyper-plane is veryhard to find. We first sample a set of points on the separatinghyper-plane, and then project sample points on 2-D planeto approximate the hyper-plane. Specifically, the sampleprocedure contains the following four steps:

1. project all training data instances onto the separatinghyper-plane;

2. calculate a bounding-box of the projections in step (1);

3. uniformly sample Nsample points in the bounding-boxon the separating hyper-plane;

4. project the Nsample sample points onto 2-D plane withAAA.

Visual Encoding To encode the data instances, threevisual channels are employed as illustrated in Figure 5.The input label of each data instance is encoded with afilled color. If the predicted label by the SVM is differentfrom its input label, a border color other than the filledcolor is employed. The shape represents whether a datainstance is a support vector; we use a rectangle for a supportvector and a circle otherwise. Furthermore, the opacity ofa data instance encodes the distance from the correspondingseparating plane.

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Channels Description

Filled Color

Border Color(optional)

Shape

Opacity

The input label of a training data instance

If the label assigned by the SVM is different from the input label

Whether it is a support vector

Distance to the correspondingseparating plane

Examples

Input label = -1

Input label = +1

Classified as +1(with the input label = -1)

No Yes

Near Far

Classified as -1(with the input label = +1)

Fig. 5 Our visual encoding scheme.

Misclassifiedinstances

Support Vectors

(a) (b)

Fig. 6 Examples of projections: (a) visual encoding applied to data points; (b)a density map composition.

For the separating hyper-plane of the SVM model, thesample points are drawn on the view as dots in grey colorwith smaller size than data points. Additionally, to visualizethe density of training data instances, a density map iscomputed and rendered. For each class, the density mapof two colors are generated separately, then two maps arecomposed on the view. Figure 6(b) shows the result.

4.1.2 Visual Exploration on Projected ScatterplotIn our visual exploration, an interactive projection control

method is provided for manipulating the direction of theselected projection. Our method is based on [13], wherethe control is similar to the “trackball control” in a high-dimensional data space. A weight is specified first foreach dimension to determine which one is going to bemanipulated, then the user can rotate the projection planeby dragging the mouse in the scatterplot view. Finallythe user’s mouse action is translated into a rotation matrixand applied to the projection matrix, thus changing thescatter-plot. However, a “gimbal lock” problem exists inthe method, which limits the rotation at the singular point.We improve their method by introducing a quaternion intothe computation of the rotation matrix to avoid this issue.

To assist the user in the exploration of the relationshipsamong multiple projections, we offer a view of multipleprojections (Figure 3(b)). Each projection glyph holds asnapshot of the interesting scatterplot inside the glyph withthe projection matrix. We define the similarity betweentwo projection glyphs as the Euclidean distance betweentwo corresponding projection matrices, i.e. ‖AAA1 − AAA2‖2.Thus, the layout of the glyphs is determined via a localaffine multidimensional projection algorithm [29]. Amongmultiple projection glyphs, the user can plan a rotation pathcontaining a sequence of projection glyphs, then a rotationanimation is generated based on interpolation betweenadjacent projections along the path [8] with a slider tocontrol the position of animation.

Two categories of exploration actions can be performedto extract knowledges from the dataset and SVM model:

Data distributions, clusters or outliers Data distributionand patterns in the projections indicate the potential locationand direction of a separating plane. The exploratorydiscovery of distributions and patterns can be performed ateach stage of the analytical process. For example, beforetraining any SVM models, the user can explore the trainingdataset to take a glance at boundaries between two classes;after an SVM model is trained, data distributions alongthe separating hyper-plane and specific patterns in supportvectors, such as outliers, may illuminate the direction offurther exploration, label manipulation or parameter tuning.

“Side views” of the separating hyper-plane When oneof the basis vectors fff 1, fff 2 is equal to the weight vector www ofthe SVM model, all the sample points will be projected intoa line in the view. This can be easily proved: let fff 1 = www,for all sample points with wwwT xxx+ b = 0, the coordinates offff 1 is a constant. Here we call it a “side view” under thiskind of projection matrix. Figure 7 shows some examples ofthe side view. Side views are useful when investigating orvalidating a trained model, because:• The boundaries of each class and gaps between them

will be shown clearly in the projection;• The distance from each data point to the hyper-plane

on a 2-D plane and the actual one in high-dimensionaldata space are in the same proportion, leading to anintuitive illustration of the spatial relations betweentraining data instances and the separating hyper-plane.

A useful exploration strategy is to start from the sideview of the separating hyper-plane. The user can rotatethe projection in a small range of angles by interactiveprojection control, in which way data distributions near thehyper-plane can be displayed and explored.

The orthogonal projection is unable to show high-dimensional data separation which may cause profoundvisual clutter. A dimension selection view (Figure 3(e))

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Distance between the data instance and the separating hyper-plane

Fig. 7 Examples of the “side view”.

is proposed for filtering out non-informative dimensions inthe classification task. In this view, three bar charts rankall the dimensions of the training data instances accordingto three ranking measures: correlation coefficients betweenthe dimension and the class labels; signal-to-noise ratio;weighting by a linear SVM [22]. The user can select themost informative dimensions, which will be highlighted inthe candidate list. After the selection is confirmed, theprojected scatterplot will be updated to apply the changes.The dimensions that are filtered out will not take part in theprojection process and the future model building process.

4.2 Visual Local SVM Building

For clarity, we use the term “global model” to representthe SVM model built with the process described in section4.1, which covers all the training data instances in thedateset. A “local model”, on the contrary, is trained on aselected subset of training data instances.

4.2.1 Visual Exploration of Decision BoundariesBefore building local SVM models, it is necessary to

perform a preliminary exploration of the decision boundaryand evaluation of complexity. First, it is necessary toexplore the data distributions, because the data distributionnear the decision boundaries is a strong indication of theboundary complexity. The user can control the projectioninteractively and inspect the patterns of boundaries betweentwo classes of data points. Additionally, the user can explorethe decision boundaries guided by the global SVM model.Although not applicable with low prediction accuracy underthe complex circumstance, the separating hyper-plane ofthe global SVM model can act as a guide to the decisionboundary. The training data instances separated into theopposite side of the hyper-plane always imply local regionscontaining non-linear boundaries, or even non-classifiablearea with mixed distributions of two classes. The user canlocate the regions in the projected scatter-plot, check thepatterns visually and make further analysis.

4.2.2 Visual Local Model Building ProcessOur visual local model building process extends the

previous global one in section 4.1. The key issue is to1) locate regions-of-interest, and 2) select proper subsetsof training data instances for each a local SVM model.We propose the following four steps to build local modelsiteratively (Figure 4).

Identification of regions-of-interest The target regionsare located via the visual exploration methods illustratedin section 4.2.1. It should be pointed out that, when somelocal models have been created, each of them but not onlythe global one can be considered as a starting point forvisual exploration and locating. Local models with differentnumbers of training data instances and ranges of coveragein high-dimensional data spaces will provide diverse levelsof details. The user can select the starting point on demand.

Selection of training data instances The training datainstances of a new local model can be selected directlyvia user interactions on the projection view. Moreover,we propose a hierarchical model creation method basedon split-and-merge operations on the created models. Alocal model can be split into two new ones by dividing itscorresponding training dataset into two subsets and trainingtwo new models on each subset. The training data instancesfrom several models can also be merged together. A newlocal model is trained on the merged dataset to replacethe existing ones. Both operations change the level ofdetails from different directions. When a model is split intoseveral multiple ones, the details of the decision boundarycan be “carved out”, while in merging operation a set ofmodels carrying much detailed information is replaced bya generalized model. The level-of-detail exploration andincremental model creation allow the user to depict thedecision boundaries and understand the distributions.

Training Once the parameter C is set for each model, thenewly created or updated local models will be re-trained inthis step.

Validation In this step, the training result is to bevalidated from two aspects: For a single local model,the same validation methods for the global model is tobe applied; for checking relations and coverage betweenmultiple models, the projection rotations between multiplemodels can be considered as indications of their positionsand directions.

After the local model building process is done, they canbe employed for predicting new data instances. A predictionalgorithm is provided based on the set of local models,where the query instances are labeled by the nearest localSVM. Algorithm 1 illustrates the prediction process.

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Algorithm 1 Prediction procedure of local SVMs

Input:The decision functions of n local SVMs, Hi(xxx), i = 1,2, . . . ,n;The training dataset of n local SVMs, XXX i, i = 1,2, . . . ,n;The query instance, xxx;

Output:Label of xxx, yi;

1: XXXknn = k nearest neighbors of xxx in⋃n

i=1 XXX i

2: inearest = argmaxi |XXXknn∩XXX i|3: yi = Hi(xxx)

4.2.3 Visualization and Interactions of MultipleModels

The statistical information of existing local SVM modelsare displayed. In particular, a matrix is used to encodethe similarity among all models in terms of the number ofshared training data instances. Each matrix cell (i, j) isdefined as:

similarity(i, j) =#(TrSet(Hi)∩TrSet(H j))

#(TrSet(Hi))

where TrSet(Hi) is the training dataset of the local modelwhose ID number is equal to i. The matrix is visualizedwith an opacity-modulated color (green in our system), asshown in Figure 3(c). The opacity of each cell is set to beits similarity.

Note that the side view best depicts the structure of alinear SVM model, rotating from the side view of a localmodel to another can depict the spatial relations betweendifferent models. This is done by taking side view snapshotsfor each model and creating a rotation path through multipleprojections.

4.3 Visual Rule Extraction

We denote a rule as a small-sized canonical subspace ofthe input high-dimensional data space that may encapsulatesome domain knowledge. The subspace is bounded by a setof dimension intervals, each of which refers to one or severalranges of a dimension. Thus, determining a rule is identicalto specifying one or several intervals in each dimension.Each rule denotes a class and assigns the correspondingclass label automatically to the data instances in the rule.

As shown in the rule extraction view (Figure 3(d)), weapply the flexible linked axes method [11] to visualizethe training data instance. The position and orientationof the dimension axes can be arranged freely. Betweenpairs of axes, the data instances are represented by parallelcoordinate plot-styled edges or scatterplot-styled dots. Thereason is that the user can choose valuable dimensionsbased on importance of dimensions and visualize the datadistributions on one dimension (on axis), two dimensions(in a scatterplot) or multiple dimensions (in a parallel

coordinate plot).The following two interaction methods are dedicated to

specify classification rules:

(a) Selecting in the projected scatterplot

(b) Brushing on axes

Rule glyphs

Fig. 8 Two ways of constructing rules.

Brushing line plots The user directly brushes the axisto select a range. Note that the number of training datainstances included in a range should be maximized, becausemore training data instances indicate the higher confidenceof the rule while classifying new instances.

Selecting points in the projected scatterplot Selectionof data points in the projected scatterplot is linked inthe rule extraction view. When selecting an interestingcluster of data instances in the projected scatter-plot, thecorresponding dots or edges are highlighted as well as theranges of their bounding-box on each axis.

After the set of selected ranges is committed, a new ruleglyph that represents the set of ranges, i.e. the rule, isdisplayed. In the rule glyph, a parallel coordinates plotis designed to summarize the dimension intervals. Thebackground color of the rule glyph encodes its expectedclass with a unique color. Next the user is given the optionto explore the relations among different rules to performfurther optimization. To express the similarity betweentwo rules, we use the force-direct layout based on theJaccard index between the sets of training data instances intwo separate rules as the similarity measure. This layoutenables a better understanding of their intersected areas. Forinstance, the glyphs that are close to each other might beredundant.

5 Case Study

5.1 System Implementation

EasySVM is primarily implemented in JavaScript forfront-end UI, which employs D3.js as graphic renderinglibrary, jQuery UI for user interface components and

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(a)

(b)

Many misclassifiedtraining data instances

(c)

Training result of the Global SVM Global SVM

Two local SVMs

Two subsets of training data instances

Initial projection result

Initial projection result

Rough gap in the large branch

Fig. 9 Analyzing the Wall-follow Robot Navigation dataset. (a) After adding the weight of projection of dimension US14, the dataset is approximately separatedinto two branches in the blue and green box. A rough gap appears in the large branch on the right side. (b) The result of Global SVM model is not applicable becausetoo many data instances are misclassified (marked in the two red circles). When increasing the projection weight on dimension US14, the projection result shows thatthe separating hyper-plane of the global SVM model is located in a different direction to the gap found in the previous step. Two separate local models are createdbased on the two branches. (c) Three classification rules are extracted based on the result of the local model built on the large branch.

Backbone.js as the MVC framework. For back-endcomputational support, we design a RESTful interface forcommunication built on Django Web Framework, and applyscikit-learn as the SVM implementation.

5.2 Wall-Following Robot Navigation Dataset

For the Wall-follow Robot Navigation dataset [16], fourmoving actions (“Move-Forward”, “Slight-Right-Turn”,“Sharp-Right-Turn” and “Slight-Left-Turn”) are to bedetermined based on the input values from 24 ultrasoundsensors (named after US0∼US23). Given a series ofsensor values, a classifier is supposed to be trained forpredicting the best action. We only use the data instancesin “Move-Forward” and “Sharp-Right-Turn” in our binaryclassification (4302 instances in total) and divide the datasetinto two parts: 50 percent as the training set, and the other50 percent as the test set.

Data exploration (Figure 9 (a)) By default, the initialprojected scatter-plot is the same as 2-D scatter-plot withonly the first two dimensions (US0, US1). The user

starts from this initial projection and performs interactiveprojection control by selecting each of the other dimensions(US2∼US23). While manipulating dimension US14, arough gap appears on a large branch on the right side, whichindicates a potential linear boundary. However, trainingdata instances of different classes in a smaller branch onthe left side are overlapping and seem impossible to belinearly separated. A snapshot is taken to support furtherinvestigation.

Global SVM model building (Figure 9 (b)) After thepreliminary data exploration, the user trains a global SVMmodel with all training data instances. However, theaccuracy on the training dataset is around 80% with differentsettings of parameter C, meaning that the dataset is notlinearly separable. In the side view, a set of wrongly-classified instances are observed to be scattered near theseparating hyper-plane.

Local SVM model building (Figure 9 (b)) The usermanipulates the dimension US14 again to investigate theprobable boundary found before, while the separating

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hyper-plane is located in a different direction. Now the userdecides to build two separated models for the two branches.After training two local SVM models, two side views showthat the two corresponding separating hyper-planes are inthe different directions and give better separation in theregions near their training datasets, which is also indicatedby the two accuracy values (around 91.3% for the modelon the smaller branch and 94.0% for the one on the largerbranch). Animated rotation among the side views of theglobal model and the two local models partially depicts therelations between three separating hyper-planes. Thus, theglobal SVM model is replaced by the two local linear ones.

Rule extraction (Figure 9 (c)) Rule extraction operationsare assisted by the two local models. The user chooses toextract rules for the local model on the large branch. Fromthe weight vector of the local model, it is obvious that thedimension US14 and US20 dominate the direction of theseparating hyper-plane. Thus the user constructs a parallelcoordinates plot between US14 and US20. The user brushesthree combinations of ranges on the two axes and generatesthree rules.

Prediction Accuracy The global linear SVM receives81%± 1.0 on the test set, while the local SVM modelsreceive 88%±3.0.

6 Discussion

In terms of non-linear SVM model building, our approachpresents an approximation method by using multiple linearmodels, which can be utilized as an interpretation tool ofthe original training dataset and a prediction tool for futureunlabelled instances. For example, each local linear SVMinterprets the boundary in a local area with its separatinghyper-plane, while a prediction can also be made with thek-NN prediction algorithm.

The trade-off between complexity and interpretabilityis important for building local SVMs. Increasing thenumber of local linear models will help to approximate thenon-linear decision boundary more accurately. However,it increases the difficulty for the user to understand thedecision boundary at the same time. Meanwhile, someof them may be redundant because it holds almost thesame information with other local models. In addition,for a training dataset containing noise around the decisionboundary, over-fitting may happen if some local modelscatch detailed information from the noises.

One promising extension of our approach is theimprovement of the scalability, including the number oftraining data instances as well as the number of dimensions.For massive data instances, clusterings method can beadopted before projection to reduce the visual clutter causedby too many data points in the 2-D plane. For the dimension

issue, we need to design a more scalable visual dimensionselection procedure to reduce the number of dimensioncandidates before the projection is performed.

7 Conclusion

In this paper, we propose a novel open-box visual analysisapproach for building SVM models. The user can performvisual exploration of the dataset and the relations betweendata instances and SVM models. Meanwhile, a visual-enhanced local linear model building approach is dedicatedto expanding the traditional linear SVM to deal with non-linear decision boundaries. Finally we provide a visual ruleextraction method to enable the user to retrieve classificationrules from the model building results.

Acknowledgements

This work was supported in part by the National 973Program of China (2015CB352503), Major Program ofNational Natural Science Foundation of China (61232012),and National Natural Science Foundation of China(61422211).

Open Access This article is distributed under the terms of

the Creative Commons Attribution License which permits any

use, distribution, and reproduction in any medium, provided the

original author(s) and the source are credited.

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First A. Author Introduction

Second B. Author Introduction

Third C. Author Introduction

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easysvm: a visual analysis approach for open-box support...

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