private void calculateMaxViewport() {
        tempMaximumViewport.set(0, MAX_WIDTH_HEIGHT, MAX_WIDTH_HEIGHT, 0);
        maxSum = 0.0f;
        for (SliceValue sliceValue : dataProvider.getPieChartData().getValues()) {
            maxSum += Math.abs(sliceValue.getValue());
        }
    }
Viewport is not really important for PieChart, this kind of chart doesn't relay on viewport but uses pixels
coordinates instead. This method also calculates sum of all SliceValues.
hellocharts-library/src/lecho/lib/hellocharts/renderer/PieChartRenderer.java