Independent gradient background for charts in subreports

Question

I have a dashboard-like report that shows several charts on one single page; these charts are divided into subreports.

i managed to put gradient background on charts using customized render class in ireport. i am using the following code for each of the chart:

public void customize(JFreeChart chart, JRChart jasperChart) {
    int offset_x = jasperChart.getX();
    int offset_y = jasperChart.getY();

    Paint p = new GradientPaint(offset_x, offset_y, Color.WHITE, offset_x, offset_y+1500, Color.BLACK);
    chart.setBackgroundPaint(p);

the problem that i am having is that the gradientbackground is shared among all subreports, which means the charts higher up on the page will have light shade of gradient paint while the charts lower down will have a darker shade of gradient.

how do i have the gradient background painted independently in each of the subreport chart?