code: def get_or_add_dPt_for_point(self, 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
docs: Return the `c:dPt` child representing the visual properties of the
          data point at index *idx*.