The unicode XML snippet for the ``<c:pt>`` elements containing the
        category names for this series.

def _cat_pt_xml(self):
        """
        The unicode XML snippet for the ``<c:pt>`` elements containing the
        category names for this series.
        """
        xml = ''
        for idx, category in enumerate(self._series.categories):
            xml += (
                '                <c:pt idx="{cat_idx}">\n'
                '                  <c:v>{cat_label}</c:v>\n'
                '                </c:pt>\n'
            ).format(**{
                'cat_idx':   idx,
                'cat_label': escape(to_unicode(category.label)),
            })
        return xml