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

[Tarquin Wilton-Jones]

Thought the rest of you might be interested in this one, since I found
the solution.

All you would need, is to include the *surveys* in your map, then use
"symbol-hide group centreline" to hide them from the render, so the map
looks like this:

map foo
  subsectionmap at subsection
  othersubsection
  anothersubsection
endmap

(Or if you need the centreline for some centreline-only section of the
survey within the part you are trying to isolate, you would need to
include a scrap or two containing a single survey station and no other
symbols from the highest and lowest parts of the cave, and then use
-attr to select and hide those stations again. Once a single station is
included from a survey, the whole survey gets used in the altitude
calculations.)

By making Therion include them, it will use their altitudes to work out
the colours, even if they will be hidden from the rendering.