Return the `c:dLbl` element representing the label of the point at
        index *idx*.

def get_or_add_dLbl_for_point(self, idx):
        """
        Return the `c:dLbl` element representing the label of the point at
        index *idx*.
        """
        matches = self.xpath('c:dLbl[c:idx[@val="%d"]]' % idx)
        if matches:
            return matches[0]
        return self._insert_dLbl_in_sequence(idx)