  We present a theory of quantum-coherent transport through a lateral p-n-p
structure in graphene, which fully accounts for the interference of forward and
backward scattering on the p-n interfaces. The backreflection amplitude changes
sign at zero incidence angle because of the Klein phenomenon, adding a phase
$\pi$ to the interference fringes. The contributions of the two p-n interfaces
to the phase of the interference cancel with each other at zero magnetic field,
but become imbalanced at a finite field. The resulting half a period shift in
the Fabry-Perot fringe pattern, induced by a relatively weak magnetic field,
can provide a clear signature of Klein scattering in graphene. This effect is
shown to be robust in the presence of spatially inhomogeneous potential of
moderate strength.
