Over at Juice Analytics they have posted a screencast showing how to make “square piecharts” like this one. (Click on the graphic to view full size.) The JA blog is full of ideas and hints on how to use Excel.

Other sources for ideas about presentation graphics, useability, and inteface design:

Edward Tufte’s web site and Q and A Forum

Jakob Nielson’s Web Site – UseIt.Com

No discussion about these topics would be complete without mentioning Kathy Sierra’s site, Creating Passionate Users. A favorite post is A Crash Course on Learning Theory.