Phase plane portrait is a very important tool to study the behavior and stability of a non-linear system.
PhasePlane(sys,tspan,icond) plots the the phase plane portrait for a general second order nonlinear system defined by the function sys(t,x).
The function plots the trajectory followed by the system for each initial condition in the cell array icond. It also plots a quiver plot to indicate the slope of any trajectory at each point of the plane.