axis([0 4.0 -1.2 1.2])
hold on
plot([1.5 1.5],[-1.2 1.2],'b--')
plot([2.5 2.5],[-1.2 1.2],'b--')
text(.75,-.6,'u=1','FontSize',15)
text(2,-.6,'u=0.5','FontSize',15)
text(3.25,-.6,'u=1','FontSize',15)
hold off