code: def xlsx_part(self, xlsx_part):
          
          rId = self._chart_part.relate_to(xlsx_part, RT.PACKAGE)
          externalData = self._chartSpace.get_or_add_externalData()
          externalData.rId = rId
docs: Set the related |EmbeddedXlsxPart| to *xlsx_part*. Assume one does
          not already exist.