docs: def f_rollup(items, times, freq):
      """
      Use :func:`groupby_freq` to rollup items
  
      :param items: items in timeseries
      :param times: times corresponding to items
      :param freq: One of the ``dateutil.rrule`` frequency constants
      :type freq: str
      """
      rollup = [np.sum(item for __, item in ts)
                for _, ts in groupby_freq(items, times, freq)]
      return np.array(rollup)