To a graph, Hausel and Proudfoot associate two complex manifolds,
\mathfrak{B}
and
\mathfrak{D}
, which behave, respectively, like moduli of local systems on a Riemann surface and moduli of Higgs bundles. For instance,
\mathfrak{B}
is a moduli space of microlocal sheaves, which generalize local systems, and
\mathfrak{D}
carries the structure of a complex integrable system.
We show the Euler characteristics of these varieties count spanning subtrees of the graph, and the point-count over a finite field for
\mathfrak{B}
is a generating polynomial for spanning subgraphs. This polynomial satisfies a deletion-contraction relation, which we lift to a deletion-contraction exact triangle for the cohomology of
\mathfrak{B}
. There is a corresponding triangle for
\mathfrak{D}
.
Finally, we prove that
\mathfrak{B}
and
\mathfrak{D}
are diffeomorphic, the diffeomorphism carries the weight filtration on the cohomology of
\mathfrak{B}
to the perverse Leray filtration on the cohomology of
\mathfrak{D}
, and all these structures are compatible with the deletion-contraction triangles.