docs: def extract_columns(data, *cols):
      """Extract columns specified in the argument list.
  
  >>> chart_data.extract_columns([[10,20], [30,40], [50,60]], 0)
  [[10],[30],[50]]
  """
      out = []
      try:
          # for python 2.2:
          # return [ [r[c] for c in cols] for r in data]
          for r in data:
              col = []
              for c in cols:
                  col.append(r[c])
              out.append(col)
      except IndexError:
          raise IndexError("data=%s col=%s" % (data, col))
      return out