  We fabricate a graphene p-n-p heterojunction and exploit the coherence of
weakly-confined Dirac quasiparticles to resolve the underlying scattering
potential using low temperature scanning gate microscopy. The tip-induced
perturbation to the heterojunction modifies the condition for resonant
scattering, enabling us to detect localized Fabry-Perot cavities from the focal
point of halos in scanning gate images. In addition to halos over the bulk we
also observe ones spatially registered to the physical edge of the graphene.
Guided by quantum transport simulations we attribute these to modified resonant
scattering at the edges within elongated cavities that form due to focusing of
the electrostatic field.
