According to this definition

Phase space - Wikipedia, the free encyclopedia

"In mathematics and physics, a phase space, introduced by Willard Gibbs in 1901[1], is a space in whichall possible statesof a system are represented, with each possible state of the system corresponding to one unique point in the phase space."

In your graph not all possible states are shown but a curve with some initial conditions.

I think that your graph a phase portrait.

The definition of phase portrait is here

Phase portrait - Wikipedia, the free encyclopedia