We study Poincaré series associated to a finite collection of divisors on i. a finite graph and ii. a certain family of metric graphs called chain of loops. Our main results are proofs of rationality of the Poincaré series and algorithms for computing it in both these cases. The main tools used in the proof of rationality are the following. For graphs, we study a certain homomorphism from a free Abelian group of finite rank to the direct sum of the Jacobian of the graph and the integers. For chains of loops, our main tool is an analogue of Lang's conjecture for Brill-Noether loci on a chain of loops and adapts the proof of rationality of the Poincaré series of divisors on an algebraic curve (over an algebraically closed field of characteristic zero). Our algorithms are based on a closer study of the objects involved in the proof of rationality, for instance, computing the fibres of certain homomorphisms and lattice point enumeration in rational polyhedra.