docs: def agg_linear_trend(x, param):
      """
      Calculates a linear least-squares regression for values of the time series that were aggregated over chunks versus
      the sequence from 0 up to the number of chunks minus one.
  
      This feature assumes the signal to be uniformly sampled. It will not use the time stamps to fit the model.
  
      The parameters attr controls which of the characteristics are returned. Possible extracted attributes are "pvalue",
      "rvalue", "intercept", "slope", "stderr", see the documentation of linregress for more information.
  
      The chunksize is regulated by "chunk_len". It specifies how many time series values are in each chunk.
  
      Further, the aggregation function is controlled by "f_agg", which can use "max", "min" or , "mean", "median"
  
      :param x: the time series to calculate the feature of
      :type x: numpy.ndarray
      :param param: contains dictionaries {"attr": x, "chunk_len": l, "f_agg": f} with x, f an string and l an int
      :type param: list
      :return: the different feature values
      :return type: pandas.Series
      """
      # todo: we could use the index of the DataFrame here
  
      calculated_agg = {}
      res_data = []
      res_index = []
  
      for parameter_combination in param:
  
          chunk_len = parameter_combination["chunk_len"]
          f_agg = parameter_combination["f_agg"]
  
          aggregate_result = _aggregate_on_chunks(x, f_agg, chunk_len)
          if f_agg not in calculated_agg or chunk_len not in calculated_agg[f_agg]:
              if chunk_len >= len(x):
                  calculated_agg[f_agg] = {chunk_len: np.NaN}
              else:
                  lin_reg_result = linregress(range(len(aggregate_result)), aggregate_result)
                  calculated_agg[f_agg] = {chunk_len: lin_reg_result}
  
          attr = parameter_combination["attr"]
  
          if chunk_len >= len(x):
              res_data.append(np.NaN)
          else:
              res_data.append(getattr(calculated_agg[f_agg][chunk_len], attr))
  
          res_index.append("f_agg_\"{}\"__chunk_len_{}__attr_\"{}\"".format(f_agg, chunk_len, attr))
  
      return zip(res_index, res_data)