[R] lattice key in blank panel

[Max Kuhn]

Somewhere I've seen an example of an xyplot() where the key was placed
in a location of a missing panel. For example, if there were 3
conditioning levels, the panel grid would look like:

34
12

In this (possibly imaginary) example, there were scatter plots in
locations 1:3 and location 4 had no conditioning bar at the top, only
the key.

I can find examples of putting the legend outside of the panel
locations (e.g to the right of locations 2 and 4 above), but that's
not really what I'd like to do.

Thanks,

Max