Just an interesting note:
The charts that you see on the dashboard in the Magento Admin area are powered by Google’s Chart API.
You can look it up here: http://code.google.com/apis/chart/
A quick and prelim. exploration of the core files show that the phtml files for this can be found here:
The controlling code is here:
Of particular interest is the class Mage_Adminhtml_Block_Dashboard_Graph
There’s also some good code to be found there that you might find useful for your own use in the site, even if its not involving using Google API to create a chart.
The controllers for this are interesting because they have some examples for ajax functionality.