Lisa, try using max() function to compute the max.
fs = 2e3;
t = (0:1/fs:1-1/fs)';
A= sin(2*pi*19*t);
B= chirp(t-0.6,61,t(end),603,'quadratic');
C=cos(2*pi*19*t);
plot(t, A, 'r-', 'LineWidth', 2);
hold on;
plot(t, B, 'g-', 'LineWidth', 2);
plot(t, C, 'b-', 'LineWidth', 2);
% Get the highest of any signal:
peakSig = max([A, B, C], [], 2);
% Get the lowest of any signal:
valleySig = min([A, B, C], [], 2);
plot(t, peakSig, 'k-', 'LineWidth', 3);
plot(t, valleySig, 'k-', 'LineWidth', 3);
grid on;
% Zoom in so we can actually see what's happening:
xlim([0, 0.04]);
legend('A', 'B', 'C', 'peakSig', 'valleySig');
If you have the valley signal in there also, you'll get this:
