I see that you want to create a plot with horizontal lines representing values above and below the mean by 2 standard deviations. Here is an example oh how we can achieve this:
A = [4 5 1];
t = [1 2 3];
plot(t,A)
hold on
avg = mean(A);
deviation = std(A);
yline(avg,"g")
yline(avg+2*deviation,"r")
yline(avg-2*deviation,"b")
hold off
Note the hold on and hold off is used to keep plotting onto the same plot. for further reading you can check out the following documentations:
