@Override
    public void onChartViewportChanged() {
        if (isViewportCalculationEnabled) {
            calculateMaxViewport();
            computator.setMaxViewport(tempMaximumViewport);
            computator.setCurrentViewport(computator.getMaximumViewport());
        }
    }
Default renderer for PieChart. PieChart doesn't use viewport concept so it a little different than others chart
types.
hellocharts-library/src/lecho/lib/hellocharts/renderer/PieChartRenderer.java