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4.2: Graphical Representation of Data

Difficulty Level: At Grade Created by: CK-12

MATLAB provides a great variety of functions and techniques for graphical display of data. The flexibility and ease of use of MATLAB's plotting tools is one of its key strengths. In MATLAB graphs are shown in a figure window. Several figure windows can be displayed simultaneously, but only one is active. All graphing commands are applied to the active figure. The command

figure(n)


will activate figure number n or create a new figure indexed by \begin{align*}n\end{align*}.

Tools for plotting

In this section we summarize some of the most commonly used functions for plotting in MATLAB.

• plot:


The

plot


function can take a large number of arguments; use the command

help plot


for further information.

plot(y)


plots the values in vector

y


versus their index.

plot(x,y)


plots the values in vector

y


versus

x


.

The plot function produces a piecewise linear graph between its data values. With enough data points it looks continuous.

• stem:


The

command

stem(y)


plots the values in vector

y


as stems from the x-axis that terminate with circles on the data values.

stem


is a natural way to plot sequences that represent discrete sequences.

stem(x,y)


plots the data sequence

y


at the values specified in

x


.

• xlabel('string')


labels the x-axis with

string


.

• ylabel('string')


labels the y-axis with

string


.

• title('string')


gives the plot the title

string


.

We illustrate these commands in Example 3.

Example 3

In this example we plot the function \begin{align*}y = x^2\end{align*} for \begin{align*}x \in [-2,2]\end{align*}. To do this, we enter the following sequence of commands:

x = -2:0.2:2;

y = x.^ 2;

figure(1);

plot(x,y);

xlabel('x');

ylabel('y=x^2');

title('Simple plot');

figure(2);

stem(x,y);

xlabel('x');

ylabel('y=x^2');

title('Simple stem plot');


This code creates Figures 4 and 5.

Figure 4

Figure 5

Some more commands that can be helpful when working with plots:

• hold on
<<p>/pre>/


hold off

Normally hold is off. This means that the plot command replaces the current plot with the new one. To add a new plot to an existing graph use


hold on

. If you want to overwrite the current plot again, use


hold off

.


• legend('plot1','plot2',...,'plot N')


The

legend command provides an easy way to identify individual plots when there are more than one per figure. A legend box will be added with strings matched to the plots.

• axis([xmin xmax ymin ymax])


Use the axis command to set the axis as you wish. Use axis on/off to toggle the axis on and off respectively.

• subplot(m,n,p)


Divides the figure window into m rows and n columns and selects the p'th subplot as the current plot, e.g

subplot(2,1,1)


divides the figure in two and selects the upper part.

subplot(2,1,2)


selects the lower part.

• grid on
<<p>/pre>/


grid off

adds or removes a rectangular grid to your plot.



Example 4

This example illustrates

hold


,

legend


,

and

axis


.

The following sequence of commands give the plots in Figure 6.

x = -3:0.1:3; y1 = -x.^2; y2 = x.^2;

figure(1);

plot(x,y1);

hold on;

plot(x,y2,'--');

hold off;

xlabel('x');

ylabel('y_1=-x^2 and y_2=x^2');

legend('y_1=-x^2','y_2=x^2');

figure(2);

plot(x,y1);

hold on;

plot(x,y2,'--');

hold off;

xlabel('x');

ylabel('y_1=-x^2 and y_2=x^2');

legend('y_1=-x^2,'y_2=x^2');

axis([-1 1 - 10 10]);


The result is shown in Figure 6.

Figure 6

Example 5

In this example we illustrate subplot and grid with the following sequence of commands:

x = -3:0.2:3; y1 = -x.^2; y2 = x.^2;

subplot(2,1,1);

plot(x,y1);

xlabel('x'); ylabel('y_1=-x^2');

grid on;

subplot(2,1,2);

plot(x,y2);

xlabel('x');

ylabel('y_2=x^2');


Now, the result is shown in Figure 7.

Figure 7

Printing and exporting graphics

After you have created your figures you may want to print them or export them to graphic files. In the "File" menu use "Print" to print the figure or "Save As" to save your figure to one of the many available graphics formats. Using these options should be sufficient in most cases, but there are also a large number of adjustments available by using "Export setup", "Page Setup" and "Print Setup".

3D Graphics

We now show a brief example of creating \begin{align*}3D\end{align*} plots. Necessary functions here are

meshgrid


and

mesh


.

In Example 6 we see that

meshgrid


produces \begin{align*}x\end{align*} and \begin{align*}y\end{align*} vectors suitable for \begin{align*}3D\end{align*} plotting and that mesh \begin{align*}(x,y,z)\end{align*} plots \begin{align*}z\end{align*} as a function of both \begin{align*}x\end{align*} and \begin{align*}y\end{align*}.

Example 6

Example: Creating our first 3D plot. We execute the following command sequence to produce the plot in Figure 8.

[x,y] = meshgrid(-3:.1:3);

z = x.^+y.^2;

mesh(x,y,z);

xlabel('x');

ylabel('y');

zlabel('z=x^2+y^2);


This code gives us the \begin{align*}3D\end{align*} plot in Figure 8.

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