Generate each ``<c:ser>`` child element in this xChart in
        c:order/@val sequence (not document or c:idx order).

def iter_sers(self):
        """
        Generate each ``<c:ser>`` child element in this xChart in
        c:order/@val sequence (not document or c:idx order).
        """
        def ser_order(ser):
            return ser.order.val

        return (ser for ser in sorted(self.xpath('./c:ser'), key=ser_order))