  A method is derived to solve the massless Dirac-Weyl equation describing
electron transport in a mono-layer of graphene with a scalar potential barrier
U(x,t), homogeneous in the y-direction, of arbitrary x- and time dependence.
Resonant enhancement of both electron backscattering and currents, across and
along the barrier, is predicted when the modulation frequencies satisfy certain
resonance conditions. These conditions resemble those for Shapiro-steps of
driven Josephson junctions. Surprisingly, we find a non-zero y-component of the
current for carriers of zero momentum along the y-axis.
