What the z-axis means from the spectrogram function?

I am reading the documentation of the spectrogram function and a do not understand a section in specific (Spectrogram and Instantaneous Frequency). The example provided in the documentation is the following:

Fs = 1000;
t = 0:1/Fs:2-1/Fs;
y = chirp(t,100,1,200,'quadratic');
spectrogram(y,100,80,100,Fs,'yaxis')
view(-77,72)
shading interp
colorbar off

The obtained result is the attached image. In the x-axis, the frequency is displayed and z-axis time is displayed, what parameter is displayed in the y-axis?

Answer 1

[Star Strider]

In image.png, the x-axis is time, the y-axis is frequency (because you told it to plot that way with the 'yaxis' argument), and the z-axis is amplitude.

The spectrogram plot is a surf plot, with the default orientation being ‘view(0,90)’. With ‘view(-77,72)’, you rotated it.

[Wendy Fernandez]

Thank you for your explanation. But, I thought z-axis was power in db. Could you explain why is amplitude?

[Star Strider]

My pleasure.

‘Amplitude’ is simply a generic term. There are many ways to express it, depending on what you are doing. You are correct that the amplitude in the power spectral density plot is power in dB.