How to draw this figure ?

Hello !

How can I draw this figure so that y(i) follows it?

This is the figure :

And this is the code to modify ( i think "ref") :

clear, clc
Ac=[0 1;0 0]; Bc=[0;1]; Cc=[1 0]; Dc=0;
[A,B,C,D]=c2dm(Ac,Bc,Cc,Dc,0.1)
ref=5;
N=600;
Q=[1 2;2 1] ; R=0.1;
P=Q;
x_ref=[ref;0];
for k=N:-1:0
    F=R+B'*P*B;
    K=inv(F)*B'*P*A;
    M=P-P*B*inv(F)*B'*P;
    P=A'*M*A+Q;
end
u=0 ; x=[0;0];
for i=1:N
    x=A*x+B*u;
    u=-K*(x-x_ref);
    y(i)=C*x;
end
plot(1:N,ref*ones(1,N),'r',1:N,y)