I personally used JFreeChart is several projects. I found it to be of very high quality.
JFreeChart

JFreeChart is a free Java class library for generating charts, including:

* pie charts (2D and 3D);
* bar charts (regular and stacked, with an optional 3D effect);
* line and area charts;
* scatter plots and bubble charts;
* time series, high/low/open/close charts and candle stick charts;
* combination charts;
* Pareto charts;
* Gantt charts;
* wind plots, meter charts and symbol charts;
* wafer map charts;

Strongly recommended.