Try this (considerably simplified version of your code):
t=linspace(0,7*pi,300);
i= [ 2 3 4 5 ];
plot(t,cos(pi*0.1*i(:)*t));
xticks(0:pi:7*pi)
xticklabels((""+(0:7))+"\pi")
legend (sprintfc('%.1f\\pi',i*0.1))
The sprintfc function is undocumented although quite useful. You can also use the compose function to do the same thing here.
