Group by supplied dimension(s) and overlay each group

        Groups data by supplied dimension(s) overlaying the groups
        along the dimension(s).

        Args:
            dimensions: Dimension(s) of dimensions to group by

        Returns:
            NdOverlay object(s) with supplied dimensions

def overlay(self, dimensions=None, **kwargs):
        """Group by supplied dimension(s) and overlay each group

        Groups data by supplied dimension(s) overlaying the groups
        along the dimension(s).

        Args:
            dimensions: Dimension(s) of dimensions to group by

        Returns:
            NdOverlay object(s) with supplied dimensions
        """
        if dimensions is None:
            dimensions = self.kdims
        else:
            if not isinstance(dimensions, (list, tuple)):
                dimensions = [dimensions]
            dimensions = [self.get_dimension(d, strict=True)
                          for d in dimensions]
        dims = [d for d in self.kdims if d not in dimensions]
        return self.groupby(dims, group_type=NdOverlay)