code: def number_format(self):
          
          number_format = self._number_format
          if number_format is None:
              return self._series_data.number_format
          return number_format
docs: The formatting template string that determines how the value of this
          data point is formatted, both in the chart and in the Excel
          spreadsheet; for example '#,##0.0'. If not specified for this data
          point, it is inherited from the parent series data object.