  We study the effect of trigonal warping on the focussing of electrons by n-p
junctions in graphene. We find that perfect focussing, which was predicted for
massless Dirac fermions, is only preserved for one specific sample orientation.
In the general case, trigonal warping leads to the formation of cusp caustics,
with a different position of the focus for graphene's two valleys. We develop a
semiclassical theory to compute these positions and find very good agreement
with tight-binding simulations. Considering the transmission as a function of
potential strength, we find that trigonal warping splits the single Dirac peak
into two distinct peaks, leading to valley polarization. We obtain the
transmission curves from tight-binding simulations and find that they are in
very good agreement with the results of a billiard model that incorporates
trigonal warping. Furthermore, the positions of the transmission maxima and the
scaling of the peak width are accurately predicted by our semiclassical theory.
Our semiclassical analysis can easily be carried over to other Dirac materials,
which generally have different Fermi surface distortions.
