code: def marks(value):
          
          for i, entry in enumerate(value):
              _assert_is_type('marks[{0}]'.format(i), entry, Mark)
docs: list or KeyedList of ``Mark`` : Mark definitions
  
          Marks are the visual objects (such as lines, bars, etc.) that
          represent the data in the visualization space. See the :class:`Mark`
          class for details.