title: Plotting a raster behind a shapefile
body: <p>How can I plot a "raster" object behind a shapefile object? Both plot fine on their own but the points don't plot over the raster:</p>
  
  <pre><code>require(rgdal)
  require(maptools)
  require(raster)
  
  myproj = "+proj=utm +zone=12 +north +ellps=WGS84 +units=m"
  shp = readShapeSpatial(fn.shp, proj4string = CRS(myproj))
  ras = raster(fn.tif)
  
  plot(ras)
  plot(shp, bg="transparent", add=TRUE)
  </code></pre>