  Let X be a tropical curve (or metric graph), and fix a base point p on X. We
define the Jacobian group J(G) of a finite weighted graph G, and show that the
Jacobian J(X) is canonically isomorphic to the direct limit of J(G) over all
weighted graph models G for X. This result is useful for reducing certain
questions about the Abel-Jacobi map Phi_p : X -> J(X), defined by Mikhalkin and
Zharkov, to purely combinatorial questions about weighted graphs. We prove that
J(G) is finite if and only if the edges in each 2-connected component of G are
commensurable over the rationals. As an application of our direct limit
theorem, we derive some local comparison formulas between g and its pullback
Phi_p^*(g) for three different natural "metrics" g on J(X). One of these
formulas implies that Phi_p is a tropical isometry when X is 2-edge-connected.
Another shows that the canonical measure on a metric graph X, defined by S.
Zhang, measures lengths on the image Phi_p(X) with respect to the "sup-norm" on
J(X).
