why doesnt muPAD draw a dotted line at the discontinuity?

![I have the initial drawing which I need to make using sums of step functions][1]

Solution:

We have 3 lines L1 ,L2 and L3 L1:y-y1=(y2-y1)/(x2-x1)(x-x1) y-1=(3-1)/(1-0)(x-0)

    Simplify(y-1=(3-1)/(1-0)*(x-0));

    solve(y-1=2*x|x=t,y);

    L1(t):=2*t+1:;

    f1(t):=u0(t):;
    f2(t):=-1*u0(t)|t=t-1:;
    S1(t):=f1(t)+f2(t):;
    L1(t):=S1(t)*L1(t):;
    plot(L1(t));

and then :

    f1(t):=3*u0(t)|t=t-1:;
    f2(t):=-3*u0(t)|t=t-2:;
    L2(t):=f1(t)+f2(t):;plot(L2(t));

    L1:y-y1=(y2-y1)/(x2-x1)(x-x1)
    Simplify(y-1=(0-1)/(3-2)*(x-2));

    solve(x+y=3|x=t,y);

    delete f1,f2;
    L3(t):=3-t:;
    f1(t):=u0(t)|t=t-2:;
    f2(t):=-1*u0(t)|t=t-3:;
    S2:=f1(t)+f2(t):;
    L3(t):=S2(t)*L3(t):;
    plot(L3(t));

And the final plot:

    L(t):=L1(t)+L2(t)+L3(t);
    plot(L(t),GridVisible);

I wonder why MuPAD doesn't show asymptote between points (2,3) and (2,1)... is it a bug ?