Gnuplot tricks

One of the freely available programs for all kind of operating systems is gnuplot - the home page for this package is http://www.gnuplot.info/

Basically, it is a scripting language that allows you to generate graphs from plaintext data through scripts. There are a wide variety of output formats, and it generates reasonably compact versions of publication-grade graphs. It includes a library to fit data and to evaluate functions, which have a syntax that is inspired by Fortran :)