docs: 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