Richard Clark, KB7QHC wrote:
"The velocities of what?"

Figure 8-32 labels the ordinate values as "relative phase velocity".
Phase velocity is defined in my electronic dictionary as:
"The velocity at which a point of constant phase is propagated in a
progressive sinusoidal wave."

In other words, pick a point on a waveform. The rate at which it moves
is phase velocity. That`s the velocity of propagation.

Best regards, Richard Harrison, KB5WZI