code: def xlsx_part(self):
          
          xlsx_part_rId = self._chartSpace.xlsx_part_rId
          if xlsx_part_rId is None:
              return None
          return self._chart_part.related_parts[xlsx_part_rId]
docs: Return the related |EmbeddedXlsxPart| object having its rId at
          `c:chartSpace/c:externalData/@rId` or |None| if there is no
          `<c:externalData>` element.