Save an image (only content, without axes or anything else) to a file using Matloptlib

I'd like to obtain a spectrogram out of a wav file and then save it to a png, but I need just the content of the image (not axes or anything else). I came across these questions
Matplotlib plots: removing axis, legends and white spaces
scipy: savefig without frames, axes, only content
I've also read the Matplotlib documentation but it seems useless and so either answers to questions above are outdated or I'm doing something wrong because simple

plt.savefig('out.png', bbox_inches='tight', pad_inches=0)

does not do what I want to achieve. Initially I tried to follow this guide but the code crashes. Then I tried this approach, but since it's outdated I modified it a little:

import matplotlib.pyplot as plt
from scipy.io import wavfile
import numpy as np

def graph_spectrogram(wav_file):
    rate, data = wavfile.read(wav_file)
    pxx, freqs, bins, im = plt.specgram(x=data, Fs=rate, noverlap=384, NFFT=512)
    plt.axis('off')
    plt.savefig('sp_xyz.png', bbox_inches='tight', dpi=300, frameon='false')

if __name__ == '__main__': # Main function
    graph_spectrogram('...')

This is what I got:
Maybe it's not visible, but there's a white border around the content (from the biggest to the smallest): left, bottom, top, right. I want the same image but just the content without anything else. How can I achieve that? I use python 3.6 and Matplotlib 2.0.2.

I think you want subplots_adjust:

fig,ax = plt.subplots(1)
fig.subplots_adjust(left=0,right=1,bottom=0,top=1)
ax.axis('tight')
ax.axis('off')

In this case:

import matplotlib.pyplot as plt
from scipy.io import wavfile
import numpy as np

def graph_spectrogram(wav_file):
    rate, data = wavfile.read(wav_file)
    fig,ax = plt.subplots(1)
    fig.subplots_adjust(left=0,right=1,bottom=0,top=1)
    ax.axis('off')
    pxx, freqs, bins, im = ax.specgram(x=data, Fs=rate, noverlap=384, NFFT=512)
    ax.axis('off')
    fig.savefig('sp_xyz.png', dpi=300, frameon='false')

if __name__ == '__main__': # Main function
    graph_spectrogram('...')