JPanel background

The easiest way to do this is to extend JPanel and override the paintComponent method.

public void paintComponent( Graphics g )
{
super.paintComponent( g );
Graphics2D g2d = (Graphics2D) g;
}
cf

Gaussian Mixture Model

漫談 Clustering (3): Gaussian Mixture Model

Source code
Multivariate Data Analysis Software and Resources
https://engineering.purdue.edu/~bouman/software/cluster/