Set the related |EmbeddedXlsxPart| to *xlsx_part*. Assume one does
        not already exist.

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