docs: def get_trend(self, item, nb=6):
          """Get the trend regarding to the last nb values.
  
          The trend is the diff between the mean of the last nb values
          and the current one.
          """
          raw_history = self.get_raw_history(item=item, nb=nb)
          if raw_history is None or len(raw_history) < nb:
              return None
          last_nb = [v[1] for v in raw_history]
          return last_nb[-1] - mean(last_nb[:-1])