[Python] Strange plt.pcolormesh Behavior: Only One Line Visible

While doing exercises with different spectrograms, I discovered some behavior in pcolormesh that I can't explain. Could you please explain what I am doing wrong or what I didn't understand?

For example, the code below shows only one line (the upper one).

import matplotlib.pyplot as plt
import numpy as np

t = np.linspace(0, 5, 301) # Ajouter un point pour les bords
f = np.linspace(-1000, 1000, 501) # Ajouter un point pour les bords

valeurs = np.zeros((500, 300))



valeurs[100, :] = 10
valeurs[400, :] = 10

plt.pcolormesh(t,f,valeurs)
#plt.pcolormesh(valeurs) #Give the same result except on axis marks.
plt.show()

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