  Let G be a graph, and let $\chi$G be its chromatic polynomial. For any
non-negative integers i, j, we give an interpretation for the evaluation $\chi$
(i) G (--j) in terms of acyclic orientations. This recovers the classical
interpretations due to Stanley and to Green and Zaslavsky respectively in the
cases i = 0 and j = 0. We also give symmetric function refinements of our
interpretations, and some extensions. The proofs use heap theory in the spirit
of a 1999 paper of Gessel.
