def get_json_history(self, item=None, nb=0):
        """Return the history (JSON format).

        - the stats history (dict of list) if item is None
        - the stats history for the given item (list) instead
        - None if item did not exist in the history
        Limit to lasts nb items (all if nb=0)
        """
        s = self.stats_history.get_json(nb=nb)
        if item is None:
            return s
        else:
            if item in s:
                return s[item]
            else:
                return None