docs: def mean_samples(data, xcol, ycollist):
      """Create a sample list that contains
      the mean of the original list.
  
  >>> chart_data.mean_samples([ [1, 10, 15], [2, 5, 10], [3, 8, 33] ], 0, (1, 2))
  [(1, 12.5), (2, 7.5), (3, 20.5)]
  """
      out = []
      numcol = len(ycollist)
      try:
          for elem in data:
              v = 0
              for col in ycollist:
                  v += elem[col]
              out.append((elem[xcol], float(v) / numcol))
      except IndexError:
          raise IndexError("bad data: %s,xcol=%d,ycollist=%s" % (data, xcol, ycollist))
  
      return out