The Control System Toolbox and System Identification Toolbox graphics functions do not generally allow you alter their properties.
The easiest work-around is to get the outputs of the functions and plot them yourself in ordinary plot (or subplot) calls:
[y,t,x] = initial(G1, initial_state);
figure(1)
plot(t, y, t, zeros(numel(t),1),':')
You can do everything with that plot that you can do with any other plot object.
