value() should accept variable line tags as well as literals
e.g. $self->{tag} would be useful to say 'the corresponding line'
It should also be able to access values created from 'codes'.

Key: icons should be central in text block, not just 1st line.
Also, the key layout is not sensitive to a change in point size.
- should it be?

Document function options:
graph
gtype
scale
key
style
shown
strict
order

