Powerful PHP Charts Library: JpGraph
JpGraph is a feature-rich PHP library for creating good looking charts.
It has support for almost any chart type including bar, line, pie, scatter, radar, field, stock and much more.
Some of the charts have the option to be created in 3D and they are all web-friendly (usually 2-3 kb).
The library is very flexible, any number of plots can be drawn in every chart, objects can be rotated, etc.
JpGraph has built-in images for plot marks including 3D rendered markers like diamonds, squares, bevels, balls and pins.
And, a very original feature, it has a pre-defined function to create flags for 200+ countries.
To sum up, the library is really an impressive one and it is free to be used in non-commercial, open-source or educational projects.