Plotting Options and Tips for Matlab

• Basic plots
• Plotting points rather than lines
• Titling and labelling axes
• Adding text to a plot
• Legends for complex plots
• Adding lines to an existing plot
• Specifying the axis limits on a plot
• Multiple plots on one page
• Including variable values in a string
• Greek letters and other symbols
• Does it have to be this hard?
• An example - with comments

• x-y plots: plot(x,y)
• semilog plots: semilogy(x,y), semilogx(x,y)
• log-log plots: loglog(x,y)

Sets of x-y data pairs can be plotted simultaneously by just adding pairs in the input list as in

   plot(x1,y1,x2,y2,...)

If X and Y have the same number of rows (M) and columns (N) OR if X is M-by-1 and Y is M-by-N then the command

   plot(X,Y)

To disable the point-to-point connections, you should provide a third argument, the line-style. For example,

   >> plot(x,y,'o')  

places a circle at the x-y locations but does not connect successive values. There are many options for plot symbols as well as line styles. See help on plot.

You can plot a line and points simultaneously (in version 5) via a combined command like

   >> plot(x,y,'o-.') 

For example, if you have just plotted your data, use the commands

   >> xlabel('This string goes to the x-axis')
   >> ylabel('This string goes to the y-axis')
   >> title('This string goes to the title of the plot')
   

If you make a mistake or want a different title or label, just re-issue the command.

1. Simple: Use gtext

   gtext('A message')

Several lines of text can be prepared for several mouse click by using a character array instead of a single string. For example,

   str = char('This is the first line','and this is #2')
   gtext(str)

If your message is a couple of lines long, you can also use a cell array to send the "wrapped" text to gtext. For example,

   str = {'This is line #1','and this is line #2'};
   gtext(str)

If you have a single-line string, you can generate an appropriately wrapped, multi-line string using textwrap:

   str = 'This is a line of characters that should be wrapped.'
   wrpstr = textwrap(cellstr,15)
   gtext(str)

Use what works for you.

2. Detailled: Use text

   xpos = {an x-coordinate value}
   ypos = {a y-coordinate value}   
   text(xpos,ypos,'A message')

Another way is to add a small box that shows each of the lines (by color or style) along with a descriptive string. In MATLAB, this is accomplished via the legend command.

You use legend after you have created a plot with several lines on it. legend keeps track of how many lines (along with their styles) are in the plot and so only asks you to provide a string for each. It then does all the work of creating a legend.

For example,

   x = 0:0.05:6;
   y1 = sin(x);
   y2 = sqrt(x);
   plot(x,y1,x,y2)
   legend('sin(x)','sqrt(x)')

   plot(...)        % create first plot
   hold on          % freeze the plot
   plot(...)        % add new lines
   hold off         % release the plot

Issuing the command

   subplot(M,N,index)

Here are some useful subplots with the (M,N,index) values given in parentheses:

In the second example above, note that different cell sizes can be drawn by controlling the "template" that subplot operates under. As is evident, there is no need to to completely use all axes in particular subdivision.

For example, say you wanted to put the date in the title of a plot that is created in an Mfile. Assuming that the plot has been created, you can give the following commands

   dateval = date                          % get the date as string
   titlestr = ['Plot created on ',dateval] % create a string
   title(titlestr)                         % create the title

For example, the following lines will create a 2-by-2 subplot of the function sin(jt) for j = 1,2,3 and 4. For each plot, the value of j is included in the title for the plot using int2str.

   t = linspace(0,7,200);
   for j = 1:4
      subplot(2,2,j)
       plot(t,sin(j*t))
       axis([0 7 -1 1])
       titlestr = ['Plot of harmonic #',int2str(j)];
       title(titlestr)
   end

Tighter control of string formatting is available through the sprintf command. Check out the help information on sprintf and its cousin, fprintf.

The plotting functions in MATLAB do a reasonably good job of setting useful axis limits based on the data you provide. However, there may be times when you want to be a bit more specific and override the default values. The axis command is available for just this operation.

Once you have created a plot, you can issue the axis command in the form

   axis([xlo xhi ylo yhi])

Scientific or engineering plots would be pretty boring if the Greek letters used in equations had to be spelled out every time. MATLAB 5.x includes the ability to render LaTex text to get things like Greek letters or exponents to appear in titles and labels. For example,

   title('Results for \beta = 3')

will render a Greek beta in the title of a plot. The "\" that preceeds the word "beta" indicates to MATLAB that the information that directly follows should be rendered as if the type-setting language LaTex were in force (try it). The range of letters (and their appropriate spellings) can be found by issuing the command

   doc text

moving to the Property List section of the page and then clicking on the link to Strings.

Exponents and subscripts require a bit more concentration. For example, to get the string

to appear in an axis label, use the command

   xlabel('Rate coefficient, K_o, in sec^{-1}')

The underscore (_) is the indication to LaTex that what follows is to be rendered as a subscript while the caret (^) is the indication to LaTex that what follows should be rendered as a superscript. The braces ({}) serve to define what the underscore or caret is to apply to.

MATLAB's implementation of LaTex is limited but very useful for most common labelling jobs. The online help will be your best guide here.

A strength of MATLAB (in my opinion, anyway) is that it permits you to control many aspects of the plots you create in a (relatively) straightforward way. However, there are times when you might want to use something a bit more interactive or flexible to create that beautiful, informative graphic.

As of version 5.2.1, MATLAB has included a function called plotedit that allows you some measure of interactive titling, labelling and axis control. You have two ways to access this command

1. Issue the command plotedit on (see the help information)
2. Select the "Show Tools" item from the "Tools" menu in a figure window (PC/Unix only)