  In this paper we analyze the effects of nonlocality on the optical properties
of a system consisting of a thin metallic film separated from a graphene sheet
by a hexagonal boron nitride (hBN) layer. We show that nonlocal effects in the
metal have a strong impact on the spectrum of the surface plasmon-polaritons on
graphene. If the graphene sheet is shaped into a grating, we show that the
extinction curves can be used to shed light on the importance of nonlocal
effects in metals. Therefore, graphene surface plasmons emerge as a tool for
probing nonlocal effects in metallic nanostructures, including thin metallic
films. As a byproduct of our study, we show that nonlocal effects lead to
smaller losses for the graphene plasmons than what is predicted by a local
calculation. We show that these effects can be very well mimicked using a local
theory with an effective spacer thickness larger than its actual value.
