jfreechart, how to paint a graph transparently black

Question

The provided link (this is the result I need) demonstrates the desired result. To achieve this, I tried to access the X y chart graph and set its background coloring to a semi-transparent color using the following code:

val plot = chart.getXYPlot(); plot.setBackgroundPaint(new Color(0x4D575656, true));

The result that came out I tried this, but it turns out this