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.

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])