Difference between revisions of "Matlab Plotting"

Revision as of 15:55, 23 August 2008

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X-Y Line Plots

Perhaps the most common plots we create as engineers are x-y line plots that show the functional dependence of one variable (y) on another (x). In MATLAB, an x-y plot is very simple to make. For example, if we want to plot on the interval (-π,π) we can do this simply as

   x = linspace(-pi,pi);  % create the x vector
   plot(x,cos(x));        % plot cos(x) versus x.

Basics

The basic syntax for plotting a function is the following

   plot( x, y );       % simplest way to plot a single function
   plot( x, y, 'abc'); % controls the line style

Here a represents the color to use when plotting this function, b represents the type of symbol to use (dots, squares, etc), and c represents the style of the line to use (solid, dashed, etc). The following three tables describe what to insert for a, b and c to control plot styles.

The following are some examples of creating plots

   x = linspace(-pi,pi);    % create 100 points between -pi and pi
   y = sin(x) .* cos(x);    % evaluate a function at each point
   plot(x,y,'rs:');         % plot a red dotted line with squares
   figure; plot(x,y,'g--'); % plot a green dashed line
   figure; plot(x,y,'ko');  % plot black circles (no line)

Note that the figure command creates a new plotting window. If you leave it out, the next plot will be overwritten.

Title and Axis Labels

Any time you create a plot, you must also label it! Axis labels (with units where appropriate) are critical. To create a title and label axes a plot in MATLAB, we use the following:

  xlabel('label text');
  ylabel('label text');
  title('title text');

Example:

   x=linspace(-pi,pi);
   plot(x,cos(x),'r--');
   xlabel('\theta');
   ylabel('cos(\theta)');
   title('A plot of the function cos(\theta) as a function of \theta');

Multiple Lines on a Graph

We often want to place multiple lines on a single plot. For example, if we want to plot the functions

on the interval (-π,π) we could do the following

   x = linspace(-pi,pi);
   f1 = cos(x);
   f2 = sin(x);
   plot(x,f1, x,f2);

Here MATLAB automatically selects the colors for the lines. If we want more control over line styles, we can specify them ourselves:

   plot(x,f1,'k-', x, f2, 'r:');

The hold on command

   x = linspace(-pi,pi);
   f1 = cos(x);
   f2 = sin(x);
   plot(x,f1,'k-');
   hold on;
   plot(x,f2,'r:');
   hold off;

Adding a Legend

When we have multiple lines on a single plot, we should add a legend to describe each line. The legend command can be used for this purpose. Its syntax is

   legend('entry 1', 'entry2', ... 'entry n');

Continuing with our previous example, we would have something like

   plot(x,f1,'k-', x, f2, 'r:');
   xlabel('x'); ylabel('f(x)');
   legend('cos(x)','sin(x)');

   plot(x,f1, x,f2);
   xlabel('x'); ylabel('f(x)');
   legend('cos(x)','sin(x)');

Log-Scale Plots

• semilogx
• semilogy
• loglog

Subplots

Fine-Tuning

• Adding a grid
• Changing axis ranges

Contour & Surface Plots

Histograms & Bar Graphs