docs: def SDAVG(self, days=45):
      """ the last 45 days average.
          計算 days 日內之平均數，預設 45 日
      """
      if len(self.raw_data) >= days:
        data = self.raw_data[-days:]
        data_avg = float(sum(data) / days)
        return data_avg
      else:
        return 0