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.

def numeric_str_val(self, date_1904=False):
        """
        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.
        """
        label = self._label
        if isinstance(label, (datetime.date, datetime.datetime)):
            return '%.1f' % self._excel_date_number(date_1904)
        return str(self._label)