Frequency axis in continuous wavelet transform graph (scale chart) in python

Question

I have an EEG signal, which I'm interested in analyzing it in both the time and frequency domains. I have already used the scipy.signal.spectrogram function, but I think that using bursts can give better results to extract the function. I tried to run a continuous wavelet transform on an artificial signal, which I created as follows:

fs = 128.0
sampling_period = 1/fs
t = np.linspace(0, 2, 2*fs)
x = chirp(t,10,2,40,'quadratic')

coef, freqs = pywt.cwt(x, np.arange(1,50),'morl', 
sampling_period=sampling_period)

and then I built the coef matrix:

plt.matshow(coef)
plt.show()

My question is how to change the scale and axis of time?