public CategoryPlot findSubplot(PlotRenderingInfo info, Point2D source) {
        if (info == null) {
            throw new IllegalArgumentException("Null 'info' argument.");
        }
        if (source == null) {
            throw new IllegalArgumentException("Null 'source' argument.");
        }
        CategoryPlot result = null;
        int subplotIndex = info.getSubplotIndex(source);
        if (subplotIndex >= 0) {
            result =  (CategoryPlot) this.subplots.get(subplotIndex);
        }
        return result;