[Therion] Colouring by altitude

[Graham Mullan]

Olly and Marco are right, of course, that consistent colouring between plan
and elevation does help in visualising the correspondemce between different
parts of the cave. This does not *need* to be by elevation (although that
can sometimes be useful) and indeed that might even cause confusion where
separate parts of the cave are at the same elevation.

We colour by map, which is a reasonable proxy for doing so by passage. The
problem is that there is not, as far as I am aware, any method of directly
choosing the colours, as Therion does that for you. That makes it important
to list individual components the same way in both plan and elevation and
that is not always possible.

The best answer would be to allow colour choice, so that the individual
draughtsman can choose exactly how he wishes to display his drawing.

I, of course, have no idea at all as to how easy it would be to implement
this.

Graham