@Override
    public void drawUnclipped(Canvas canvas) {
        super.drawUnclipped(canvas);
        final Viewport currentViewport = computator.getCurrentViewport();
        final float left = computator.computeRawX(currentViewport.left);
        final float top = computator.computeRawY(currentViewport.top);
        final float right = computator.computeRawX(currentViewport.right);
        final float bottom = computator.computeRawY(currentViewport.bottom);
        previewPaint.setAlpha(DEFAULT_PREVIEW_TRANSPARENCY);
        previewPaint.setStyle(Paint.Style.FILL);
        canvas.drawRect(left, top, right, bottom, previewPaint);
        previewPaint.setStyle(Paint.Style.STROKE);
        previewPaint.setAlpha(FULL_ALPHA);
        canvas.drawRect(left, top, right, bottom, previewPaint);
    }
Renderer for preview chart based on ColumnChart. In addition to drawing chart data it also draw current viewport as
preview area.
hellocharts-library/src/lecho/lib/hellocharts/renderer/PreviewColumnChartRenderer.java