> In the trellis panel functions, or in the direct plot/image/lines etc
> functions, you plot in data coordinates. The main thing that the plot
> and spplot methods in package sp control is the aspect ratio. How else
> do data coordinates differ from geographical coordinates when it comes
> to plotting?
Exactly. In fact I think that a grid panel might make it possible to
insert an image backdrop behind spplot(), which was the original question
from Dylan - it would make a nice student project in visualisation.
Something like an sp.image() for sp.layout=, but using its own palette and
being painted over by the "real" data.