Set the value of ./c:x@val to *offset* and ./c:xMode@val to "factor".

def horz_offset(self, offset):
        """
        Set the value of ./c:x@val to *offset* and ./c:xMode@val to "factor".
        """
        self.get_or_add_xMode().val = ST_LayoutMode.FACTOR
        self.get_or_add_x().val = offset