code: def numeric_str_val(self, date_1904=False):
          
          label = self._label
          if isinstance(label, (datetime.date, datetime.datetime)):
              return '%.1f' % self._excel_date_number(date_1904)
          return str(self._label)
docs: The string representation of the numeric (or date) label of this
          category, suitable for use in the XML `c:pt` element for this
          category. The optional *date_1904* parameter specifies the epoch used
          for calculating Excel date numbers.