code: def horz_offset(self, offset):
          
          if offset == 0.0:
              self._remove_manualLayout()
              return
          manualLayout = self.get_or_add_manualLayout()
          manualLayout.horz_offset = offset
docs: 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.