def end_at(x, y=nil)
      use_two_cell_anchor unless @anchor.is_a?(TwoCellAnchor)
      @anchor.end_at x, y
      @anchor.to
    end
def end_at(x, y=nil)
      use_two_cell_anchor unless @anchor.is_a?(TwoCellAnchor)
      @anchor.end_at x, y
      @anchor.to
    end
noop if not using a two cell anchor
 @param [Integer] x The column
 @param [Integer] y The row
 @return [Marker]