How to calculate the slope of a linear chirp signal

I know that calculating the 1st derivative gives the slope of the chirp signal but the chirp signal varies with time. so I get many values .Please explain me how to calculate the slope for this chirp signal...

f01 = 0; f02 = 100;           % 0 - 150 Hz
f11 = 300; f12 = 400;         % 350 - 500 Hz
Fs = 2*max(f11,f12);
t = 0:1/Fs:1000*(1/Fs)- (1/Fs);
t1 = t(end);
y1 = chirp(t,f01,t1,f11,'linear');