docs: def SD(self, days=45):
      """ Standard Deviation.
          計算 days 日內之標準差，預設 45 日
      """
      if len(self.raw_data) >= days:
        data = self.raw_data[-days:]
        data_avg = float(sum(data) / days)
        data2 = []
        for x in data:
          data2.append((x - data_avg ) ** 2)
  
        return math.sqrt(sum(data2) / len(data2))
      else:
        return 0