Designing artificial signals
In the code below I have manually created chirps where the frequency does not increase linearly, but behaves in a cosine-shape, as suggested by Atul Ingle . This waveform resembles the shape of the lines in the spectrogram nicely.

fs = 5000;
x0 = 1;
t = -x0:1/fs:x0;
yi = cos(t).*sinc(t/1.5).*cos(2*pi*500*(1.8-cos(2.2*t)).*t); 
yj = cos(t).*sinc(t/1.5).*cos(2*pi*500*(0.85-0.5*cos(2.3*t)).*t); 
subplot(2,2,1)
plot(t,yi+yj)
subplot(2,2,2)
plot(t,yj)
subplot(2,2,3)
spectrogram(yi+yj,512,511,256,fs,'yaxis')
subplot(2,2,4)
plot(t,yi)