The MRPL graphs as a downward sloping line

The firm wears two hats remember, one as a buyer of labor and one as a seller of products. If the firm sells its product in a perfectly competitive product market, then its marginal revenue, MR, is equal to product price, P, and is constant regardless the level of Q. So MRPL = P · MPL. Because MPL is downward sloping, multiplying by a positive constant simply alters its height, but preserves its shape.

If the firm has market power in setting its price, then MR < P, and MR diminishes as Q increases. Multiplying two data series both of which diminish as Q increases simple means that the result, MRPL, will diminish even faster as Q increases.

Thus, the MRPL faced by a firm takes price as given is more elastic than the MRPL faced by a firm that sets price, but MRPL is downward sloping in both cases.