According to this definition
Phase space - Wikipedia, the free encyclopedia
"In mathematics and physics, a phase space, introduced by Willard Gibbs in 1901, is a space in which all possible states of 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