JFreeChart charting tool
JFreeChart is a 100% Java library for creating professional looking charts in Java applications. It is intended for both client side and server side applications. It supports creating charts in multiple formats such as PNG and JPEG. JFreeChart supports pie charts (2D and 3D), scatter plots, time series charts, bar charts (horizontal and vertical, regular and stacked), line charts, candlestick plots, Gantt charts, combined plots, thermometers and dials.
The only problem is that the developer guide for JFreeChart is not free! Currently it costs $48.75 per developer for the developer guide (I wonder how that is enforced). There is also a single site license worth $487.50.