def _ZeroPoint(self, chart):
    """Get the zero-point if any bars are negative."""
    # (Maybe) set the zero point.
    min, max = chart.GetDependentAxis().min, chart.GetDependentAxis().max
    out = {}
    if min < 0:
      if max < 0:
        out['chp'] = 1
      else:
        out['chp'] = -min/float(max - min)
    return out
Get the zero-point if any bars are negative.
mapreduce/lib/graphy/backends/google_chart_api/encoders.py