title: R: overlay plot on levelplot
body: <p>I have a raster file 'airtemp' and a polygon shapefile 'continents'. I'd like to superimpose the 'continents' on 'airtemp', so the boundary of 'continents' is visible on top of 'airtemp'. I plot the raster file by <code>levelplot</code> (lattice). I read the polygon by <code>readShapeSpatial</code> (maptools) first and then <code>plot</code>. </p>
  
  <p>The problem is <code>levelplot</code> and <code>plot</code> have different scales. <code>Plot</code> tends to have smaller frame. Sorry I don't have a reproducible sample, but I feel this is a rather common issue for geophysicists. I've found a similar question here:</p>
  
  <p><a href="http://r.789695.n4.nabble.com/overlaying-a-levelplot-on-a-map-plot-td2019419.html" rel="noreferrer">http://r.789695.n4.nabble.com/overlaying-a-levelplot-on-a-map-plot-td2019419.html</a></p>
  
  <p>but I don't quite understand the solution.</p>