GNUPlot is an awesome free plotting program that works on any platform (mac, windows, linux, etc.), that combines power and ease-of-use. GNUPlot can output to any format you can imagine, (pdf, ps, eps, gif, png, direct latex code, x11, wxt, aquamac, etc.) and it can also be viewed interactively (i.e. zooming in and out with mouse clicks and spinning around 3D plots.).

GNUPlot is actively being developed and is used across multiple scientific fields, not just astronomy. It is installed on the cluster, though not the latest version. If you want the latest version, Jeff has it installed in his directory and will be more than happy to show you how to install it. You could also get it from the GNUPlot site. LANL has a good step-by-step intro which goes into a wealth of detail.