f[x]: sets the trajectory in the x direction.
f[y]: sets the trajectory in the y direction.
size: sets the number of pixels in the E and B panels.
dt: sets the time step.
E scale min: sets the minimum value of the electric field vectors shown.
E scale max: sets the maximum value of the electric field vectors shown.
B scale min: sets the minimum value of the magnetic field lines shown.
B scale max: sets the maximum value of the magnetic field lines shown.