Top view (normal to H-plane) vertical axis is 'a' dimension. The Vector magnetic field (Hz and Hx) is shown in blue.
The underlying contours are equipotentials for the respective field components. These are what you see when the FDM or exact solutions for Hz and Hx are projected onto the the Z axis of a 2 independent, 1 dependent variable GRAPH axis (not the z/length direction of the waveguide!)