Why Figure E9-2 is an elevation plot: the horizontal line at the left/right edge is the horizon (0 degrees) and angles are measured upward from it toward zenith (90 degrees), with radial grid lines every 15 degrees. The main lobe's maximum lies midway between the horizon and the first 15 degree radial, so the peak response occurs at 7.5 degrees. That low takeoff angle is typical of a horizontal antenna a wavelength or so above real ground, and it is exactly what you want for long-haul DX.