Phase Portraits are created by generating Phase Plots for different starting points. Phase plot can be created using ODEPHAS2 or ODEPHAS3 in matlab as a value for 'OUTPUTFCN' property in ODESET while creating options object. Following is an example.
function pdecaller()
options = odeset('OutputFcn',@odephas2);
for i=-4:4
for j= -4:4
[t,y] = ode45(@vdp1,[0 20],[i;j],options); hold on;
end
end
end
function dydt = vdp1(t,y)
dydt = [y(2); (1-y(1)^2)*y(2)-y(1)];
end