Basic Plots

gnuplot is a very powerful program, and has many options for specifying how the plots turn out. However, in most of the plots you will do, you will simply be looking for the information contained within the plot, and the gnuplot defaults will be quite acceptable. The advanced options will be covered in the next section. The basic command to get information from to gnuplot is the plot command. See Section $[*]$ for detailed reference information. plot normally takes either a single matrix, a list of matricies, or a string as its argument. If you wish to, you may also add a second argument, which is the plot process number (see above). If the argument is a string, the string (without the surrounding double quotes) is passed directly to gnuplot. This was used above, and will be used again when we see the advanced plotting features. If the argument is a single matrix, then first column is taken as the independant variable, and the second and higher columns are plotted against it. Here is an example showing it:

> x = 0:2*pi:0.01;
> y = sin(x);
> A = [x',y'];
> // Check that A is what we wanted
> show(A)
   name:      A     
   class:     num   
   type:      real  
     nr:      629   
     nc:      2     
> // Two columns is what I had in mind. Plot to the screen to check it
> plot(A);
> // That was what we wanted. Now dump it to a file.
> setterm("latex");
> plot("set output \"sine.plot\" ");
> plot(A);

The plot looked something like this: $\begin{center}\vbox{\input{sine.plot} }\end{center}$ If the matrix only has one column, then the index values are used as the independant variable, and the column vector is plotted as the dependent variable. Here is a example showing this feature:

> B = rand(10,1) * 20;
> // Still in latex mode, need new file to avoid over-write
> plot(" set output \"random.plot\" ");
> plot(B);

Which will produce output like: $\begin{center}\vbox{\input{random.plot} }\end{center}$ The third type of input is using a list of matricies. This is a very flexible tool, though it has little point unless the matricies have different first columns. In this case, the two matricies will be plotted on the same graph, over the domains specified by the first column. Here is an example:

> x = -pi:pi:0.1;
> w = 0:2*pi:0.1;
> C = [x', cos(x)'];
> D = [w', sin(w)'];
> plot(<<C;D>>);
> // Just as expected...
> setterm("latex");
> plot("set output \"CosNSin.plot\" ");
> plot(<<C;D>>);

Which will produce output like: $\begin{center}\vbox{\input{CosNSin.plot} }\end{center}$ In that example, the independent variables had a partly common domain, though there is no reason why this has to be so, as gnuplot is capable of accepting non-intersecting domains. If you want to have two different plots at the same time, you need to specify a plot process number. Here is an example of how to produce three graphs on a system capable of displaying several windows, such as X windows:

> x = -pi:pi:0.1;
> // The first plot defaults to plot process zero
> plot([x',cos(x)']);
> plot([x',sin(x)'],1);
> // Now combine the two
> plot([x',cos(x)',sin(x)'],2);

The last command shows how to plot multiple curves on a single plot without using a list — remember that each column after the first is treated as a dependant variable to the first column. The effect of this is best seen by actually trying it on a system capable of displaying multiple plots. After you have produced multiple plots (requiring multiple copies of the gnuplot program to be run), you may wish to clean up the screen and free up some resources, so you need to kill some of the plots. You could always exit , but that is not always desirable. So the pclose function is provided. Section $[*]$ has all the details, so suffice to say that you can kill off a plot process by specifying its plot process number as an argument to pclose. With all those plot process numbers floating around, it is easy to lose track of how each one is set up, and which files it is using (or would use). So the showplot function is provided. This function is documented in Section $[*]$ . A typical use of showplot is:

>  x = -pi:pi:0.1;
>  plot([x',cos(x)',sin(x)'],1);
>  plot([x',cos(x)',2*sin(x)'],2);
> showplot(2);
        Plot List 2
                Term:                   X11
                Output:                 stdout
                Tmp Files:               rlab-tmpf-2-1-1 rlab-tmpf-2-2-1
                HardCopy:               postscript
                HardCopy Files:         rlab-tmp-hrdf.2
> showplot(1);
        Plot List 1
                Term:                   X11
                Output:                 stdout
                Tmp Files:               rlab-tmpf-1-1-1 rlab-tmpf-1-2-1
                HardCopy:               postscript
                HardCopy Files:         rlab-tmp-hrdf.1