superimposing a heatmap on an image

[Francesco Potortì]

I know how to plot over an image: I can superimpose lines and graphical
objects on a background.

This time I need to superimpose a heatmap (for example one obtained with
imagesc from a data matrix) transparently over a background image.

After some tricks, I managed to obtain what I want:  I did it
by taking the background image, then taking my data matrix, converting
it to RGB using a colormap and merging it with the background image.

But I obtain an image, not the same thing as imagesc does.  The axes are
not what I want, and I can not plot over it using the right coordinates.

Is there a way to call imagesc on the background image, hold on, then
imagesc again?

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