JFreeChart chart = ... //Get the chart with your first dataset.
CategoryPlot plot = chart.getCategoryPlot();
plot.setDataset(1, anotherDataset); //The '1' at the first parameter is
//the index of dataset.
//Thus, '1' for the second dataset.
... //some other settings to the plot, with index=1.
plot.setRenderer(1, someRenderer);
Тогда мы можем получить график с этим графиком.