Personally, I really like gnuplot. There are other ideas in the nodes below.

• Creating Graphs - Best module to use
• State of the Ch(art) in Perl
• Graphics Module
• making a scientific chart
• Perl module for chart drawing?
• How to creata a Line Graph
• Pixel-based Plotting in Perl?
• Line Graph Image Manipulation
• Better plotting than GD::Graph?
• Real time data graphs.
• drawing (visualizing) directed graphs
• How to print/draw a network graph
• Really slick Charts and Graphs
• Portable simple line graph module?
• Charting Modules
• Professional, "a-la-OpenOffice", 3D graphs
• develop animation and graphics
• creating graphs
• (YA) Q: re "which graphics module?"

I'm also looking at SVG for a project of mine (An SVG Case Study: Integrated, Dynamic Avalanche Forecasting has some nice examples).

Updated 2011-01-13 with nodes more recent than Graphics Module