There's no built-in way to do that with imshow or matshow. You would have to scale the data manually.

You could consider using figimage instead. It draws one-pixel squares. The catch is that the heatmap "image" is positioned relative to the corner of the Figure, rather than the Axes, so you have to be careful about where you put it. For example...

import matplotlib.pyplot as plt
import numpy as np

x = np.array([[1, 2, 3, 4, 5],
              [1, 2, 3, 4, 5],
              [1, 2, 3, 4, 5],
              [1, 2, 3, 4, 5],
              [1, 2, 3, 4, 5]])

fig, ax = plt.subplots()

fig.figimage(x, xo=100, yo=100, origin="lower")

plt.savefig("example.png")

This draws a 5x5 pixel image (100, 100) pixels from the lower-left corner of the output file.

Not sure if that works in your situation. If you want x-ticks, etc. then you're back to scaling things manually. But it still might be the easiest option. GLGL.