[R] Key for custom lattice panel function

[Hadley Wickham]

Hi all,

I've created a custom lattice panel function for levelplot - instead of 
representing z by colour, it plot circles with radius proportional to z 
(in the style of the map plots of Jacques Bertin).  I'm happy to email 
an example graph to anyone interested.

The problem is now to create a key for the plot.  This is difficult 
because all of the other lattice plots convey information through point 
colour, shape, and texture, not point size.  For this reason (and having 
looked at the code) I don't think I can shoehorn $key or $colorkey to 
produce the type of key that I want. 

I'm using grid.circles() with native units that have been scaled in the 
same way as in panel.levelplot.

Can anyone offer any suggestions as to how to create this key?

Hadley