Set the value of ./c:manualLayout/c:x@val to *offset* and
        ./c:manualLayout/c:xMode@val to "factor". Remove ./c:manualLayout if
        *offset* == 0.

def horz_offset(self, offset):
        """
        Set the value of ./c:manualLayout/c:x@val to *offset* and
        ./c:manualLayout/c:xMode@val to "factor". Remove ./c:manualLayout if
        *offset* == 0.
        """
        if offset == 0.0:
            self._remove_manualLayout()
            return
        manualLayout = self.get_or_add_manualLayout()
        manualLayout.horz_offset = offset