Overshoot/undershoot in non-minimum phase system
As it's a unit step, let's consider just from t=[0,+inf)

At t=0, you have y=1 (could be found considering s->inf in the transfer function).

At t=inf, you have -1/2 (could be found considering s->0 in the transfer function).

So the non minimum phase zero in this system, considering a unit step in t=0, makes the output go from zero to positive (t=0, y=1) before coming back to negative (t=+inf, y=-1/2) in steady state. I believe your graph represents just that.