integral convolution in matlab ?

salmo allikm warhmat allah wabrakato

 i am still learning matlab  and trying to do  integral convolution for this  two signals 
X(t)=2*(U(T)-U(T-2))
H(t)=3*(U(T)-U(T-1))
CONV(X,H)
i think there is something wrong the output all are zero althougth x,h plot are right
this code

t= -20:0.001:20;
y=heaviside(t);
v=heaviside(t-2);
x=2*(y-v);
plot(t,x)
axis([-4 8 04]);
h=3*[heaviside(t)-heaviside(t-1)];
plot(t,h)
axis({-3 4 0 4]);
y=conv(x,h);
plot(t,y(1:length(t)))
axis({-3 4 0 4]);

thanks