How to change the resolution of a spectrogram?

I have written a code which used to plot the spectrogram of a signal. This line was to get the spectrogram:

[s,f2,t] = spectrogram(xc,win,noverlap,FFT_LENGTH,fs,'yaxis');

Then I used imagesc to plot the spectrogram: imagesc(t,f2,20*log10(abs(s)));

Everything is working fine except the following thing:

I don't know how to change the scale/resolution for the thing circled by a red line [looks like steps] to make it looks like a straight line (not like steps)? (please see the attached spectrogram)

[Jan]

Modify the values for win, noverlap and FFT_LENGTH.

[Sultan Al-Hammadi]

that's sounds correct