Feedback Control for Computer Systems

Appendix B. Creating Graphs with Gnuplot

Gnuplot is an open source program for plotting data and functions. It is intended primarily for Unix/Linux systems, although versions for Windows and the Mac exist as well.

Basic Plotting

When started, gnuplot provides a shell-like interactive command prompt. All plotting is done using the plot command, which has a simple syntax. To plot a function, you would type (at the gnuplot prompt):

plot sin(x)

By default, gnuplot assumes that data is stored in white-space separated text files, with one line per data point. To plot data from a file (residing in the current directory and named data), you would use the following:

plot "data" using 1:2 with lines

This assumes that the values used for the horizontal position (the x values) are in the first column, and the values for the vertical position (the y values) are in the second column. Of course, any column can be used for either x or y values.

Gnuplot makes it easy to combine multiple curves on one plot. Assume that the data file looks like the one shown in Figure B-1 (left). Then the following command would plot the values of both the second and the third column, together with a function, in a single graph (Figure B-1, right):

plot "data" u 1:2 w lp, "data" u 1:3 w lp, 5*exp(-5*x)

Here we have made use of the fact that many directives can be abbreviated (u for using and w for with) and have also introduced a new plotting style, linespoints (abbreviated lp), which plots values as symbols connected by lines. ...