Return the `c:tx` element for this data label, with its `c:rich`
        child and descendants, newly created if not yet present.

def _get_or_add_tx_rich(self):
        """
        Return the `c:tx` element for this data label, with its `c:rich`
        child and descendants, newly created if not yet present.
        """
        dLbl = self._get_or_add_dLbl()

        # having a c:spPr or c:txPr when a c:tx is present causes the "can't
        # save" bug on bubble charts. Remove c:spPr and c:txPr when present.
        dLbl._remove_spPr()
        dLbl._remove_txPr()

        return dLbl.get_or_add_tx_rich()