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Two-dimensional Plots

ELEC 206

Prof. Siripong Potisuk

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The Plot Command

The simplest command for 2-D plotting

Syntax: >> plot(x,y)

The arguments x and y are vectors (1-D arrays)

which must be of the same length, but any type

x contains data points along the horizontal axis

y contains data points along the vertical axis

When executed, a figure window pops up containing the figure with the graph of y vs. x

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The Figure Window Every time a plot command is issued, MATLAB

checks if a figure window is currently opened

If one is, MATLAB will erase the plot in it and

draw a new one.

If none is, a new figure window pops up and a

graph drawn in it

If multiple windows are opened, MATLAB will

draw in the latest one

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The Figure Window To specify the figure window number, use

>> figure(n) where n is an integer indicated

the figure number

If figure window n exists, makes it the active

window

If figure window n doesn't exist, creates it and

makes it the active window

If n is omitted, the new figure window will be

labeled “figure n” where n is the integer greater

than the current active figure number by 1 4

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If after issuing a plot command and

no figure window appears, it is

buried.

Click on the figure window icon on

the task bar to make the window appear

Closing Figure Window

Use close command to close figure windows

- close closes active Figure Window

- close(n) closes Figure Window n

- close all closes all open Figure Windows

Use clf command to clear contents of the current

figure window

Use clf(n) command to clear contents of the nth

figure window

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>> yr = 1988:1994;

>> sales = [8 12 20 12 18 24 27];

>> plot(yr,sales)

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>> x = 1:10;

>> y = x.^2

>> plot(x,y)

Blue is the default line color

Points connected by straight lines

with no point markers

Both axes are linear

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Optional Arguments

The vector x can be omitted

Syntax: >> plot(y)

MATLAB creates a plot of the vector y on the

vertical axis against a set of numbers from 1 to

n where n is the number of elements of y on

the horizontal axis, i.e., a plot of y against its

indices

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>> y=[2.1 2.1 2.1 1 0 -1 -2.1 -2.1 -2.1];

>> plot(y)

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Important parts of a 2-D plot

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Changing Default Settings

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Line Specifiers

define style and color of lines, and marker types

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Property Name and Values

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>> yr = 1988:1994;

>> sales = [8 12 20 12 18 24 27];

>> plot(yr,sales,’--r*’,’linewidth’,2,...

‘markersize’,12)

1988 1989 1990 1991 1992 1993 19948

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Exporting the Figure

To copy the entire figure window into another

program, e.g., Word, PowerPoint

1. Click on the figure window to make it current

window

2. Press ALT+PRNTSCRN

3. Paste into other application

To copy just the plot area into another program,

e.g., Word, PowerPoint

1. In figure window, select Edit, then Copy Figure

2. Paste into other application 18

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Superimposing Multiple Graphs

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Plot two or more graphs on the same plot as

follows (e.g., for three graphs)

plot(x,y,u,v,t,h)

Plots y vs. x, v vs. u, and h vs. t

Vectors of each pair must be of the same size

Different pairs can be of different sizes

Can use line specifiers by putting in triplets (x-

data, y-data, specifier), e.g.,

plot(x,y,'-b',u,v,'--r','t,h,'g:')

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x=[-2:0.01:4];

y=3*x.^3-26*x+6; yd=9*x.^2-26; ydd=18*x;

figure(2)

plot(x,y,'-b',x,yd,'--r',x,ydd,':k')

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Superimposing Multiple Graphs

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Normally, each time you execute plot, MATLAB erases a previous plot and draws a new one.

To change this behavior: – Draw the first graph with plot

– Issue the command hold on

– Call plot for each of the remaining graphs

– Issue the command hold off

Graphs drawn after hold on are added to plot

Graphs drawn after hold off erase plot

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x=[-2:0.01:4];

y=3*x.^3-26*x+6; yd=9*x.^2-26; ydd=18*x;

figure(2)

plot(x,y,'-b') % create the first

% graph of y

hold on

plot(x,yd,'--r') % superimpose the

% second graph of yd

plot(x,ydd,':k') % superimpose the

% third graph of ydd

hold off

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Superimposing Multiple Graphs

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The line command adds additional graphs to an

existing plot

Syntax:

line(x,y,'PropertyName','PropertyValue')

Example:

line(x,y,'linestyle','--','color','r',

'marker','o')

adds graph drawn with dashed red line and

circular markers to current plot

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x=[-2:0.01:4];

y=3*x.^3-26*x+6; yd=9*x.^2-26; ydd=18*x;

figure(2)

% create the first graph of y

plot(x,y,'-b')

%

% superimpose the second graph of yd

line(x,yd,'lineStyle','--','color','r')

%

% superimpose the third graph of ydd

line(x,ydd,'lineStyle',':','color','k')

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Making Multiple Plots

in the Same Figure

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The subplot command divides the current

active Figure Window into m rows and n columns

of subplots

Syntax: subplot(m,n,p)

p in the subplot command

refers to the numbering of

subplots starting from left

to right and row by row

from top to bottom

The Subplot Command

If subplots don't exist, subplot(m,n,p)

creates them and makes subplot p the

current subplot

If subplots exist, subplot makes subplot

p the current one

When subplot defines current subplot,

next plot and formatting commands draw

in this current subplot

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x=[-2:0.01:4];

y=3*x.^3-26*x+6; yd=9*x.^2-26; ydd=18*x;

figure(3)

subplot(3,1,1); plot(x,y,'-b') % graph of y

subplot(3,1,2); plot(x,yd,'--r') % graph of yd

subplot(3,1,3); plot(x,ydd,':k') % graph of ydd

Logarithmic Scales

One or both axes

Used to display data with large range of values

Used to make some functional relationships more

apparent, e.g. y = 10(2x+3) is a straight line on a

semilog plot

Can use line specifiers and property-name property-value pairs as in plot

On logarithmic axis, make sure all data is > 0

because otherwise log is undefined

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MATLAB commands for log plots

y = logspace(a,b,n) generates a row vector y of n

points between decades 10a and 10b. If n omitted, default to

50 points y = logspace(a,pi) generates the points between 10a

and pi, which is useful for digital signal processing where

frequencies over this interval go around the unit circle.

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