Gnuplot
My preferred method of formatting graphical information is an open-source package called Gnuplot. This is a page of useful Gnuplot links and tutorials.
You are of course welcome to use whatever you like, as long as it produces high-quality vector-format (not bitmapped) graphics.
Many other references and tutorials can be located by searching Google for gnuplot tutorial. Note that the version of gnuplot installed on the computers in PhSc 123 is 4.0, and most of these tutorials are for version 3.7.x. This is generally not a problem as most of the older commands are still supported in 4.0, but it is worth noting the differences because they may not be supported in later versions.