The essence of the lower graph is that the relationship between modulation frequency and frequency deviation is very different when you use a PM modulator vs when you use an FM modulator. When FM modulating, the frequency deviation remains constant no matter what modulating frequency is used. However, when using a phase modulator, the frequency deviation increases with modulation frequency. This lower graph therefore shows that the FM modulator differs from the PM modulator by a mathematical differentiation (or conversely integration). Which one? Depends on your point of view. If you want an FM modulator, you can add an integrator before the PM modulator. If you want a PM modulation, you can add a differentiator before the FM modulator. (now, did I get that right, or is that backwards? I often confuse myself on this).