code: def meanAndStdDev(self, limit=None):
          
          if limit is None or len(self.values) < limit:
              limit = len(self.values)
          if limit > 0:
              mean = sum(self.values[-limit:]) / float(limit)
              sumSq = 0.
              for v in self.values[-limit:]:
                  sumSq += (v - mean) * (v - mean)
              return mean, math.sqrt(sumSq / limit)
          else:
              return None
docs: return the mean and the standard deviation optionally limited to the last limit values