The unicode XML snippet for the ``<c:pt>`` elements when category
        labels are numeric (including date type).

def _cat_num_pt_xml(self):
        """
        The unicode XML snippet for the ``<c:pt>`` elements when category
        labels are numeric (including date type).
        """
        xml = ''
        for idx, category in enumerate(self._series.categories):
            xml += (
                '                <c:pt idx="{cat_idx}">\n'
                '                  <c:v>{cat_lbl_str}</c:v>\n'
                '                </c:pt>\n'
            ).format(**{
                'cat_idx':     idx,
                'cat_lbl_str': category.numeric_str_val(self._date_1904),
            })
        return xml