[R-sig-Geo] Multiple legends in spplot

[Lee Hachadoorian]

Trying to produce spplot with a point layer on top of a polygon layer. 
Looks like this can be accomplished using plot() (see for example http://
www.asdar-book.org/code.php?chapter=2&figure=7), but I want to use spplot 
for the typical reasons (i.e, to produce several polygon maps showing 
related attributes using the same quantitative classes).

That problem statement might be enough to point me to an example 
somewhere, since I assume I'm not the first person who has wanted to do 
this, but I haven't been able to find anything in the help or the 
archives. Apologies if I'm missing something obvious.

More specifically, I would like something that looks like this figure 

http://r-spatial.sourceforge.net/gallery/#fig04.R

except instead of a north arrow, the right should have a colorkey for the 
polygon object. Using standard data:

library(sp)
library(lattice)

#CREATE SpatialPolygonsDataFrame OF RIVERBED
data(meuse.riv)
meuse.lst = list(Polygons(list(Polygon(meuse.riv)), "meusee.riv"))
spdfMeuse = SpatialPolygonsDataFrame(
	SpatialPolygons(meuse.lst),
	data.frame(anything = 1, row.names = "meusee.riv")
)

#CREATE SpatialPoints OF SOIL SAMPLES
data(meuse)
coordinates(meuse) = c("x", "y")

#THIS PLOTS POLYGONS AND POINTS WITH A colorkey FOR THE POLYGONS
spplot(spdfMeuse,
	sp.layout = list("sp.points", meuse, pch = as.integer(meuse$soil))
)

#THIS PLOTS POLYGONS AND POINTS WITH A LEGEND FOR THE POINTS
spplot(meuse, "soil", 
	key.space = list(x = 0.1, y = 0.93, corner = c(0,1)),
	sp.layout = list("sp.polygons", spdfMeuse, fill = "lightblue")
)

So I can combine both layers, but I haven't been able to figure out how 
to get both legends. Figure 4 from the R-spatial gallery (referenced 
above) uses the legend argument to add a North arrow. Since xyplot() 
specifies that the legend argument is necessary to produce multiple keys, 
I'm assuming the answer must make use of it. But trying to adapt the code 
for Figure 4 as:

spplot(spdfMeuse,
	sp.layout = list("sp.points", meuse, pch = as.integer(meuse
$soil)),
	legend = list(inside = list(fun = mapLegendGrob(meuse, "soil"), x 
= 1, y = 0.1, corner = c(0,0)))
)

yields:

Error in SpatialPolygons2Grob(obj, fill) : 
  object is not of class SpatialPolygons
Calls: spplot ... .local -> append -> mapLegendGrob -> 
SpatialPolygons2Grob

sessionInfo() below the sig.

Regards, 
Lee Hachadoorian
PhD Student, Geography
Program in Earth & Environmental Sciences
CUNY Graduate Center

> sessionInfo()
R version 2.12.1 (2010-12-16)
Platform: x86_64-pc-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_US.utf8          LC_NUMERIC=C                
 [3] LC_TIME=en_US.utf8           LC_COLLATE=en_US.utf8       
 [5] LC_MONETARY=en_US.utf8       LC_MESSAGES=en_US.utf8      
 [7] LC_PAPER=en_US.utf8          LC_NAME=en_US.utf8          
 [9] LC_ADDRESS=en_US.utf8        LC_TELEPHONE=en_US.utf8     
[11] LC_MEASUREMENT=en_US.utf8    LC_IDENTIFICATION=en_US.utf8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] lattice_0.19-13    RColorBrewer_1.0-2 RSQLite_0.9-2      
sp_0.9-66         
[5] RPostgreSQL_0.1-6  DBI_0.2-5          rkward_0.5.4      

loaded via a namespace (and not attached):
[1] grid_2.12.1  tools_2.12.1