[Therion] Colouring an elevation based on surveys that are not being displayed

[Tarquin Wilton-Jones]

> It is quite easy to turn a PDF into a JPG, especially on Linux. Once
> that is done, then any decent photo editor can crop it. The legend will
> be a problem. I suppose you could crop out two sections (the legend and
> the cave portion you want) and paste them into a single image.

Thanks for the suggestion.

For me, this is all very easy indeed, and if it were only me that
mattered, I would probably go down this route (as I have done with
wkhtmltopdf to get a modified cavelist into a survey). Even removing the
legend part is very easy with the various Therion options. But my
intention is to be able to share my surveys, source and all (they are
going to be one of the open source licences). Sadly, I cannot expect
another user to have to install and configure ImageMagick, and make
adjustments when adding new cave passage to a survey which shifts the
positions of things.

This is why I am hoping for a solution that works entirely within
Therion, if at all possible.

Still hoping for "altitude-relative caveMP at cave", which would be so much
easier for both my use case, and the use cases that I can see others
wanting :)