2D plot of a matrix with colors like in a spectrogram

Question

How to plot, with Python, a 2D matrix A[i,j] like this:

• i is the x-axis
• j is the y-axis
• A[i,j] is a value between 0 and 100 that has to be drawn by a colour (ex: 0=blue, 100=red)

Is there a Python function for that?

(NB: I don't want a function that does the spectrogram for me, such as specgram, because I want to compute the FFT of the signal myself; thus I only need a function that plots a matrix with colors)

Answer 1

Let Z be the array, here is what I finally use:

plt.imshow(np.transpose(Z), extent=[0,4.2,0,48000], cmap='jet',
           vmin=-100, vmax=0, origin='lowest', aspect='auto')
plt.colorbar()
plt.show()

Notes:

• 'jet' is the colormap that is seen in the question's image, see also these colormaps

• setting origin='lowest' has the same effect than replacing np.transpose(Z) by np.transpose(Z)[::-1,]

• vmin, vmax give the scale (here from 0 to -100 dB in the example)

• extent gives the limits of the x-axis (here 0 to 4.2 seconds) and y-axis (0 to 48000 Hz) (in this example I'm plotting the spectrogram of a 4.2 second-long audio file of samplerate 96Khz)

• if aspect='auto' is not set, the plot would be very thin and very high (due to 4.2 vs. 48000 !)