title: Overplotting two SpatialPolygonsDataFrames with spplot
body: <p>I have a bunch of data which I've plotted at the county level, <a href="https://stackoverflow.com/questions/12682212/how-to-get-spplot-lattice-to-not-draw-borders-around-polygons">without borders</a>.  I'd like to add in state boundaries.  I have a state shapefile (polygons), but <code>spplot</code> doesn't seem to have any way to add on top of the previous map.  Is there any way to do this without rewriting the panel function to take two SPDFs (which seems pretty specialized for what is likely a problem other people have)?</p>
  
  <p>Here's a reproducible example:</p>
  
  <pre><code>library(sp)
  Srs1 = Polygons(list(Polygon(cbind(c(2,4,4,1,2),c(2,3,5,4,2)))), "s1")
  Srs2 = Polygons(list(Polygon(cbind(c(5,4,2,5),c(2,3,2,2)))), "s2")
  
  county &lt;- SpatialPolygonsDataFrame( SpatialPolygons(list(Srs1,Srs2)), 
                                    data.frame( z=1:2, row.names=c("s1","s2") ) )
  
  SrsA &lt;- Polygons(list(Polygon(cbind(c(3,5,5,1,3),c(3,4,6,5,3)))),"sA")
  state &lt;- SpatialPolygonsDataFrame( SpatialPolygons(list(SrsA)),
                                    data.frame( z=1,row.names="sA" ))
  
  spplot( county, zcol="z",col=NA )
  spplot( state, add=TRUE ) # Note the add=TRUE does nothing here, but that's the spirit of what I want to accomplish
  </code></pre>