Basics of GNUplot

GNUplot is an easy to use plotting program available on Unix/Linux or wherever you have GNUtools installed.

To start, GNUplot, at the prompt type gnuplot
To quit GNUplot, type quit

If you have a textfile with data, for example mydata.txt that looks like:
1      4.91      2.64
2      3.82      2.90
3      3.01      3.45
4      2.15      4.16
5      2.95      4.99
6      3.61      5.82
7      4.33      5.11
8      4.80      4.51
9      4.91      3.80
10    4.95      1.60

You can plot the data in columns 1 and 2 by typing
plot "mydata.txt" using 1:2 w l
(w l = with lines; you can use w p = with points or w lp = with lines and points -- whatever you like)

If you want to plot columns 1 and 3 without losing the plot of columns 1 and 2, you use replot by typing
replot "mydata.txt" using 1:3 w l

There's all sorts of other things you can do. To find out type help which gives you access to the in-built manual.