Shaded area in graph

Hi everyone,

I want to create a vertical shaded area when my variable a>0. So far, I have created the plot with the use of ttplot function (by Peter Perkins - exccellent graph illustration :)), so that:

time = datetime(1950,1:100,1);
y = randn(100,27);
% tt = array2timetable(y);
tt = array2table(y);
tt.time = time';
ttplot(tt)

I want to create a vertical shaded figure wherever the y variable is over zero. Do I do this inside the ttplot function or on the code, below the last line above?

Thanks in advance!