Depending on the plot resolution, it could be difficult to see easily.
Try this:
time = linspace(0, 20);
temp = 20+10*sin(2*pi*time/15);
figure
plot(time, temp)
hold on
patch([time fliplr(time)], [temp fliplr(temp)+0.1], [0.8 0 0], 'EdgeColor','none') % Plot +0.1 C Band
hold off
grid
ylim([0 35])
