Openchart2, based on the original JOpenChart library by Sebastian Müller, provides a simple, yet powerful, interface for Java programmers to create two-dimensional charts and plots.  The library features an assortment of graph styles, including advanced scatter plots, bar graphs, and pie charts.  New features are added constantly to improve the power of the library.

Best of all, Openchart2 is free!  The library is licensed under the GNU Lesser GPL, allowing for its inclusion in proprietary software (as long as modifications to the library are released).  Furthermore, the developer documentation is available to all.