How can I find a section of a graph?

I want to highlight a percentage of the maximum amplitude along the wave. What function will allow me to do this?

Answer 1

[Chunru]

% plot the curve
t = 0:.1:5;
x = cos(2*pi*3*(t-2.5))./(1+(t-2.5).^2*2);
plot(t, x, 'b-');
% peak around 2.4-2.6
idx = find(t>2.4 & t<2.6);
hold on
plot(t(idx), x(idx), 'y-', 'LineWidth', 2)