code: 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());
          }
      }
docs: 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.