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.

def cat_pts(self):
        """
        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.
        """
        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)]