Although, many libraries are available in java to create graphs but they are not easy to use. Specially I find it very difficult to download a .tar.gz from sourceforge.net and install it.
And many of us already know how to use gnuplot for creating graph, and are familiar with is simplicity. If not then this link is very useful giving info about to install it and use it. It requires a shell command in linux and a dos command in windows(I m not sure abt windows I m assuming :)) for use. But if you want to run it in java you should be able to run shell command or dos command in java. It's not a big deal you can download a sample java programme for this purpose the comments in programme are self explainatory.