Return the related |EmbeddedXlsxPart| object having its rId at
        `c:chartSpace/c:externalData/@rId` or |None| if there is no
        `<c:externalData>` element.

def xlsx_part(self):
        """
        Return the related |EmbeddedXlsxPart| object having its rId at
        `c:chartSpace/c:externalData/@rId` or |None| if there is no
        `<c:externalData>` element.
        """
        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]