How to save the contour plot with .png format in python

Question

I want to save contour in python. I can't do it very well. I used plt.savefig() but image is empty. why?

!pip install tftb
import numpy as np
import matplotlib.pyplot as plt
import scipy.io as sio
from tftb.generators import atoms
import tftb
import cv2

mat = sio.loadmat('/content/drive/MyDrive/z5_25.mat')
signal = mat['z']
z = signal.T

images_dir = '/content/drive/MyDrive/image'

for i in range(122):
wvd = tftb.processing.WignerVilleDistribution(z[i])
wvd.run()
fig = plt.figure()
plt.rcParams['figure.figsize']=(1.5,1.5)
wvd.plot(kind = 'contour')
plt.savefig(f"{images_dir}/fig5_25_{i}.png")
plt.show()