Based on the information provided, we can't tell what is wrong.
We have a simple example program, LinePlotSalesVolume that demonstrates the BufferedImage class. The file image is saved using the main menu's File | Save Image option. Have you verified that the example program works ?
If so, you should substitute you own graph for the one in that example. What are the results ?
Our own example does not use the default constructor. Instead the code looks like:
BufferedImage savegraph = new BufferedImage(chartview, fileimageformat);
savegraph.Render();
savegraph.SaveImage(filename);