As it happens, the original internal "spec" for this example **specifically** states that it should demonstrate explicit control of the display region (though obviously that was not clear from here in GitHub !).

Despite this, I did investigate it, but as I looked I got more and more doubtful about this approach..
My understanding is that the key purpose of the extended functionality in "set_extents" is that the region can be defined in a different coordinate system.  So much so, in fact, that the default crs is **not** the display crs (which I had not even realised until yesterday) :
>  "If no crs is given, the extents’ coordinate system will be assumed to be the Geodetic version of this axes’ projection."

(by "Geodetic version", it basically means that the passed region arguments are taken as lats+lons)
What this actually does is possibly open to interpretation, as obviously a simple square region in one crs is not simple in another one, but it seems to mean that the resulting display is of minimal size to show the whole of the target region.

So, I tried using the main data region to define the display area:  This does simplify the code nicely, by removing constants and function arguments for the region controls, but the resulting plots are a bit undesirable as they come out in rather different in sizes and shape.  Even more awkwardly, in order to get the whole source data to display correctly it is necessary to add bounds in calculating the region, and even then the extreme points get slightly lost, so a little margin is desirable.

So overall, I'm feeling that this is getting too messy, and this just may not be a very good place to introduce this method.