display side coord systems and aspect ratios

Using examples/MapProjectionDisplay as a (very useful!) starting point, I
have finally learnt how to do display side coord systems.  Now I can plot
a grid of data and its projection (and thus basemaps, grid lines) on a
display.  But...

if I change the aspect ratio, which I did to show that my grid of model
data is rectangular rather than square, then my display is not right. For
example, the method used in the examples/MapProjectionDisplay to track the
lat/lon, ie, finding the 'ray' from Swing's pixel coords, fails if a
non-square aspect ratio is in place (at least I think it is failing?)

Furthermore (related?), if I alter the aspect ratio, my 'grid' of data
still fits in the box (my grid is defined by the 'getDefaultMapArea' of
the MapProjection), but the box now contains more/less of the background
map, which is the OUTLUSAM file and is thus lat/lon.

PS: is there another way to locate the display values (ie the -1 -> 1 for
Display.XAxis, Display.YAxis) given the mouse coords?  I note that the
mouse drag with middle button down produces on the canvas the values for
all scalar mapped types.  Is this using the find ray method?

Any help appreciated.