jpgraph   (link)

http://www.aditus.nu/jpgraph/
http://www.aditus.nu/jpgraph/jpg_features.php
JpGraph is an OO class library for PHP >=4.3.0. (Recommended: >= 4.3.8) The only other hard requirement is that the PHP installation suppports the GD library which most modern PHP installations do.

JpGraph makes it easy to draw both "quick and dirty" graphs with a minimum of code and complex professional graphs which requires a very fine grain control.

The library assigns context sensitive default values for most of the parameters which minimizes the learning curve. The features are there when you need it - they don't get in your way. The library also comes wih very extensive documentation.