Heatmap is too Thin in Figure

How do I make this fill the figure or at least make it square?

figure('visible', 'off');
h = histogram2(x,y,50);
figure('visible', 'on');

imshow(heatmap,[], 'XData', h.XBinEdges, 'YData', h.YBinEdges, 'InitialMagnification', 'fit');
axis on;
ax = gca;
ax.YDir = 'normal';

colormap(cmap_green);
c = colorbar;
c.Label.String = 'Average Predicted vs Actual NOx';

xlabel('Engine Speed');
ylabel('Torque');

Answer 1

[J. Alex Lee]

imshow() asserts 1:1 aspect ratio to show the image, so you can manually change the axes Position property after setting DataAspectRatioMode property to "auto"