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