code: def interpolate(value):
          
          if value.value:
              _assert_is_type('shape.value', value.value, str_types)
              if value.value not in PropertySet._valid_methods:
                  raise ValueError(value.value + ' is not a valid method')
docs: ValueRef : string, line interpolation method to use
  
          Possible values for ``area`` types are `'linear'`,
          ``'step-before'``, ``'step-after'``, ``'basis'``, ``'basis-open'``,
          ``'cardinal'``, ``'cardinal-open'``, ``'monotone'``. ``line`` types
          have all values for ``area`` as well as ``'basis-closed'``,
          ``'bundle'``, and ``'cardinal-closed'``.
  
          Only used if ``type`` is ``'area'`` or ``'line'``.