[R-sig-Geo] Plotting point labels along with points using spplot sp.layout list

[reeves at nceas.ucsb.edu]

Hello:

Im working on a method to create multilayer raster/vector plots using
spplot() and supporting functions. The question is: how do I efficiently
add display list instructions for plotting the point labels that works
for different sized point vectors?

Here is a working 'brute force' approach (I have omitted the string parsing
code that produces the point labels) that creates the plot and labels the
first two points only:

library(sp)
library(rgdal)
library(maptools)
#
# read files
#
Counties  <- readShapePoly("PugetSoundCountiesClp2.shp")
psImg     <- readGDAL("PugetSoundSub1.img")
Centroids <- readShapePoints("PSCentroidPointShape")

len = length(Centroids at coords)
LatLongs = Centroids at coords
dim(LatLongs) = c(len/2,2)
#
# spplot 'sp.layout' list entries (demote polygons to spatial lines).
#
points <- list("sp.points", Centroids,  pch = 21,col="green")
Counties_lines <- as(Counties, "SpatialLines")
polys <- list("sp.lines", Counties_lines, col="white")
greys <- grey(0:256 / 256)
#
# Assume: ss[] is a char vector of point labels, one per Centroid element..
# display list entries for first two points
#
pt1 = list("sp.text",c(LatLongs[1,1],LatLongs[1,2]),ss[1],col="white",pos=2)
pt2 = list("sp.text",c(LatLongs[2,1],LatLongs[2,2]),ss[2],col="white",pos=2)
#
# I could create these for all 'n' list elements I suppose, but then the
code is not general.
#
grob2 = spplot(psImg, "band1", col.regions=greys,  
sp.layout=list(points,pl1,pl2,polys),cuts=length(greys),
colorkey=FALSE,scales=list(draw=TRUE))
#
plot(grob2)

Something tells me Im doing this the hard way...
I could construct the arguments to list() dynamically with string building
code, and then call() or eval() the command, but isnt there a much simpler
way to 1) tell sp.layout to add the point labels or 2) add the labels to
the
plot in a separate command? I have tried separate text() and
update(trellis.last.object()) commands to no avail.

Suggestions?

Regards, Rick Reeves / NCEAS