'Return:
- 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)'

def get_json_history(self, item=None, nb=0):

    s = self.stats_history.get_json(nb=nb)
    if (item is None):
        return s
    elif (item in s):
        return s[item]
    else:
        return None
