You didn't specify what the y value of your graph needs to be, but you can take the information in this post and adjust it as needed. I also reconfigured some of your code a bit.
figure(1)
title('Max Temperature','fontsize',20)
xlabel('n° sensor','fontsize',15)
ylabel('Temperature [°]','fontsize',15)
hold on
grid on
for i=1:18
plot(i,Max_Temp(1,i),'o')
end
x = 1:18;
y = ones(1,length(x))*200; %I just made up a value of 200. Put whatever your threshold is here.
plot(x,y,'-k')
hold off
