[t w]=ode45('rk',[0 40], [-1 1 -1 0]);
for i=1:size(w(:,1))
    plot(w(i,1),w(i,3),'+',t(i)/5,0,'o')
    axis([-2 8 -1 1]);
    F(i) = getframe;
end
movie(F) 