It's not really a chirp if you want to have the frequencies vary more or less randomly up and down. But anyway, to make a signal with random frequencies:
t = linspace(0, 100, 400);
amplitude = 3;
% Make random frequencies.
frequency = .04 + .04 * rand(1, length(t));
% Construct waveform.
y = amplitude * cos(2 .* pi .* frequency .* t);
plot(t, y, 'b-');
grid on;
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0, 0, 1, 1]);
