public void actionPerformed(ActionEvent event) {
                // test image plot:
                float[][] image_data = new float[100][100];
                for(int i=0; i<100;i++)
                {
                    for(int j=0;j<100;j++)
                    {
                        if(flip)
                            image_data[i][j] = -(float)(i*100.+j);
                        else
                            image_data[i][j] = (float)(i*100.+j);
                    }
                }
                flip = !flip;
                plot1.updateData(image_data);
            }