[R-sig-Geo] spplot with two rasters

[hadley wickham]

>  > 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.

Another approach would be to develop a better interface between
spatial data and ggplot2, which provides much better support for layer
compositing (and in general for modifying a plot after you have
created it).  There are some simple examples at
http://had.co.nz/ggplot2/geom_polygon.html and
http://had.co.nz/ggplot2/coord_map.html, but I don't know enough about
the problems of spatial data to be able to do it all myself.

Hadley


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