  We present a theoretical analysis of unidirectional interface states which
form near $p$-$n$ junctions in a graphene monolayer subject to a homogeneous
magnetic field. The semiclassical limit of these states corresponds to
trajectories propagating along the $p$-$n$ interface by a combined
skipping-snaking motion. Studying the two-dimensional Dirac equation with a
magnetic field and an electrostatic potential step, we provide and discuss the
exact and essentially analytical solution of the quantum-mechanical
eigenproblem for both a straight and a circularly shaped junction. The spectrum
consists of localized Landau-like and unidirectional snaking-skipping interface
states, where we always find at least one chiral interface state. For a
straight junction and at energies near the Dirac point, when increasing the
potential step height, the group velocity of this state interpolates in an
oscillatory manner between the classical drift velocity in a crossed
electromagnetic field and the semiclassical value expected for a purely snaking
motion. Away from the Dirac point, chiral interface states instead resemble the
conventional skipping (edge-type) motion found also in the corresponding
Schr\"odinger case. We also investigate the circular geometry, where chiral
interface states are predicted to induce sizeable equilibrium ring currents.
