Return the `c:dPt` child representing the visual properties of the
        data point at index *idx*.

def get_or_add_dPt_for_point(self, idx):
        """
        Return the `c:dPt` child representing the visual properties of the
        data point at index *idx*.
        """
        matches = self.xpath('c:dPt[c:idx[@val="%d"]]' % idx)
        if matches:
            return matches[0]
        dPt = self._add_dPt()
        dPt.idx.val = idx
        return dPt