usage: y = chirp(t [, f0 [, t1 [, f1 [, form [, phase]]]]])

Evaluate a chirp signal at time t.  A chirp signal is a frequency
swept cosine wave.

t: vector of times to evaluate the chirp signal
f0: frequency at time t=0 [ 0 Hz ]
t1: time t1 [ 1 sec ]
f1: frequency at time t=t1 [ 100 Hz ]
form: shape of frequency sweep
'linear'      f(t) = (f1-f0)*(t/t1) + f0
'quadratic'   f(t) = (f1-f0)*(t/t1)^2 + f0
'logarithmic' f(t) = (f1-f0)^(t/t1) + f0
phase: phase shift at t=0

Example
specgram(chirp([0:0.001:5])); # linear, 0-100Hz in 1 sec
specgram(chirp([-2:0.001:15], 400, 10, 100, 'quadratic'));
soundsc(chirp([0:1/8000:5], 200, 2, 500, "logarithmic"),8000);

If you want a different sweep shape f(t), use the following:
y = cos(2*pi*integral(f(t)) + 2*pi*f0*t + phase);