Return the byte stream of an Excel file formatted as chart data for
        the category chart specified in the chart data object.

def xlsx_blob(self):
        """
        Return the byte stream of an Excel file formatted as chart data for
        the category chart specified in the chart data object.
        """
        xlsx_file = BytesIO()
        with self._open_worksheet(xlsx_file) as (workbook, worksheet):
            self._populate_worksheet(workbook, worksheet)
        return xlsx_file.getvalue()