code: def cat_pts(self):
          
          cat_pts = self.xpath('./c:ser[1]/c:cat//c:lvl[1]/c:pt')
          if not cat_pts:
              cat_pts = self.xpath('./c:ser[1]/c:cat//c:pt')
  
          cat_pt_dict = dict((pt.idx, pt) for pt in cat_pts)
  
          return [cat_pt_dict.get(idx, None)
                  for idx in range(self.cat_pt_count)]
docs: Return a sequence representing the `c:pt` elements under the `c:cat`
          element of the first series in this xChart element. A category having
          no value will have no corresponding `c:pt` element; |None| will
          appear in that position in such cases. Items appear in `idx` order.
          Only those in the first ``<c:lvl>`` element are included in the case
          of multi-level categories.