Thoughts from a 21st century educator.
There's a PHP library called 'Image_Graph' which allows you to create very nice looking graphs fairly easily using PHP functions, and simplifies the job of creating complicated functions. Unfortunately this isn't terribly Web 2.0, so I've created a very basic GUI which allows a user to enter in values and produce a histogram from those values. The plan is to allow more types of graphs to be created, but a histogram seemed like a relatively easy place to start. The labels on the x-axis aren't quite mathematically correct yet (for a histogram, they are fine for a bar graph) so this isn't much than a curio at the moment.
You can test it out here.