Grace is a nice program for graphing all sorts of data.
It is free and can be obtained from http://plasma-gate.weizmann.ac.il/Grace/ and installed quite easily (especially if you use the binary versions).
Once installed, it is useful to set the GRACE_HOME environment variable:
>export GRACE_HOME=/usr/local/grace # or wherever it happens to be
and include the directory that contains the executable in your path:
>export PATH=$PATH:$ GRACE_HOME/bin
The documentation can be found at the above web site.