def GetMinMaxValues(self):
    if not self.stacked:
      return super(BarChart, self).GetMinMaxValues()

    if not self.data:
      return None, None  # No data, nothing to do.
    num_bars = max(len(series.data) for series in self.data)
    positives = [0 for i in xrange(0, num_bars)]
    negatives = list(positives)
    for series in self.data:
      for i, point in enumerate(series.data):
        if point:
          if point > 0:
            positives[i] += point
          else:
            negatives[i] += point
    min_value = min(min(positives), min(negatives))
    max_value = max(max(positives), max(negatives))
    return min_value, max_value
Get the largest & smallest bar values as (min_value, max_value).
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/graphy/graphy/bar_chart.py/BarChart.GetMinMaxValues