The unicode XML snippet for the ``<c:lvl>`` elements containing
        multi-level category names.

def _lvl_xml(self, categories):
        """
        The unicode XML snippet for the ``<c:lvl>`` elements containing
        multi-level category names.
        """
        def lvl_pt_xml(level):
            xml = ''
            for idx, name in level:
                xml += (
                    '                  <c:pt idx="%d">\n'
                    '                    <c:v>%s</c:v>\n'
                    '                  </c:pt>\n'
                ) % (idx, escape('%s' % name))
            return xml

        xml = ''
        for level in categories.levels:
            xml += (
                '                <c:lvl>\n'
                '{lvl_pt_xml}'
                '                </c:lvl>\n'
            ).format(**{
                'lvl_pt_xml': lvl_pt_xml(level),
            })
        return xml