docs: def median(data, freq_col=1):
      """Compute the median of the <freq_col>'th column of the values is <data>.
  
  >>> chart_data.median([(10,20), (20,4), (30,5)], 0)
  20
  >>> chart_data.median([(10,20), (20,4), (30,5)], 1)
  5.
      """
  
      nr_data = _nr_data(data, freq_col)
      median_idx = nr_data / 2
      i = 0
      for d in data:
          i += d[freq_col]
          if i >= median_idx:
              return d
      raise Exception("??? median ???")