: Plot spatial data with spplot and maps

I have made a plot with ssplot, using a SpatialPointsDataFrame. The
content is quite simple, as I have 9 grid points with lon/lat
coordinates and 9 values attached to these coordinates. They are in a
square area of 3 by 3 gridboxes.
I would like to lay a map from maps() over these values, but when I try
this, the grids of the maps  do not overlap with the grids I have
already created with spplot().

You?re trying to mix ?base? graphics and ?grid? graphics, which doesn?t work.
?spplot? use ?lattice? graphics (based on ?grid?), so you?ll have to modify
its ?panel? function. Here?s a very simple example.

BTW, I recommend reading the ?lattice? book to understand how to use ?panel? 
functions: http://lmdvr.r-forge.r-project.org/figures/figures.html

library(sp)
library(lattice)
library(maps)
library(maptools)
n=100

d=data.frame(x=rnorm(n,0,20),y=rnorm(n,0,20),z=1:n)
xl=range(d$x)
yl=range(d$y)
coordinates(d)=~x+y
spplot(d, panel=function(...) {
  panel.fill("lightblue3")
  wmap=map("world", fill=TRUE, plot=FALSE,
           xlim=xl, ylim=yl, resolution=0)
  wmap[c("x", "y")]=mapthin(wmap[c("x", "y")],1)
  wmap.sp=map2SpatialPolygons(wmap, wmap$names)
  sp.polygons(wmap.sp,fill="khaki", col=NA,lwd=2)
  panel.pointsplot(...)
}, xlim=xl, ylim=yl )

The reason for the slow drawing is that each polygon is plotted separately,
and the map polygon also contains areas outside ?xl? and ?yl?. (This is easy
enough to fix by merging and clipping the polygons.)

Karl Ove Hufthammer