You can add xlims, ylims keyword arguments to plot!():
plot!(plot1, 1:100, ones(100) .* 4, label="", xlims=xlims(plot1), ylims=ylims(plot1))
You can add xlims, ylims keyword arguments to plot!():
plot!(plot1, 1:100, ones(100) .* 4, label="", xlims=xlims(plot1), ylims=ylims(plot1))