Remove white border when using subplot and imshow in python (Matplotlib)

Look at my example it may help you:

import numpy as np
import matplotlib.pyplot as plt

def save_image(data, filename):
    sizes = np.shape(data)     
    fig = plt.figure()
    fig.set_size_inches(1. * sizes[0] / sizes[1], 1, forward = False)
    ax = plt.Axes(fig, [0., 0., 1., 1.])
    ax.set_axis_off()
    fig.add_axes(ax)
    ax.imshow(data)
    plt.savefig(filename, dpi = sizes[0], cmap='hot') 
    plt.close()

data = np.random.randint(0, 100, (256, 256))
save_image(data, '1.png')

Little modification to above answer:

def save_image(data, filename):
    fig = plt.figure(figsize=(1, 1))
    ax = plt.Axes(fig, [0., 0., 1., 1.])
    ax.set_axis_off()
    fig.add_axes(ax)
    ax.imshow(data, cmap="gray")
    fig.savefig(filename, dpi=data.shape[0]) 
    plt.close(fig)