The Franklin Institute
Frankin Institute Online: Group Graphing
This site from The Franklin Institute explores how to make a simple graph using a spreadsheet to portray survey data. It also gives a set of interesting sports-related web sites so that students can get statistical inforamtion.
Wolfram Research
Wolfram Math World: Pie Chart
This page briefly discusses the use of pie charts (or circle graphs) in representing data. Provides detailed information for understanding.
Other
Abp: P Ie Charts
This site from Accelerators and Beam Physics Group produces pertinent information regarding pie charts. Shows an example of a pie chart and breaks it up for easy learning.