Why Figure E9-2 plots the radiation in a vertical plane: the angle scale is measured from the horizon upward, and the lobes are stacked above the ground with a null straight down/along the ground, which is the signature of an elevation (vertical plane) pattern. Reading it gives you things like the takeoff angle of the main lobe (about 7.5 degrees in this figure) and the front-to-back ratio. Antenna modeling programs produce both this vertical slice and a horizontal slice for every antenna.