[Kwant] Velocities

Hello, Can you point me to the correct direction of where to look at the documentation to figure out how to make a vorticity plot in kwant? Essentially, I want to attach leads to a graphene flake. I want to find the vorticity around each closed hexagon. Which command of kwant should I look at to figure out the velocity between two adjacent atoms so that I can calculate the vorticity in the end? Also, kwant.solvers.default.wave_function(some_sys, some_energy) Where is some_energy a reference to? An excitation energy relative to what? Thanks, Ben