Here is an example:
fs=1000; % sampling rate (Hz)
t=(0:1000)/fs; % time vector for x (s)
x=chirp(t,50,1,300)+chirp(t,350,1,75); % x= sum of up-chirp and down-chirp
% next: compute STFT
[s,f,t1]=stft(x,fs,'FrequencyRange','onesided','FFTLength',64,'Window',hann(64));
contour(t1,f,abs(s)) % plot contours of STFT
colorbar
Good luck.
