title: Add XY points to raster map generated by levelplot
body: <p>I have <code>raster</code> maps which are generated using the <code>raster</code> package in R. These raster layers can be visualized using the <code>rasterVis</code> package's <code>levelplot</code> function:</p>
  
  <pre><code>levelplot(rasterstack, layout=c(1, 2), 
            col.regions=colorRampPalette(c('darkred', 'red3', 'orange2', 'orange', 
                                           'yellow', 'lightskyblue', 'steelblue3', 
                                           'royalblue3', 'darkblue')))
  </code></pre>
  
  <p>Now, I would like to add some z values defined by xy cordinates to the levelplot map. The dataframe containing z values has 4 columns. Columns 1 &amp; 2 contain x &amp; y coordinates, column 3 contains z values for map 1 in layout(1, 1) and column 4 for layout(1, 2).</p>
  
  <p>The points per map should be added such that if z &lt; 0.05, <code>pch=2</code> and if z > 0.05, <code>pch=3</code>. </p>
  
  <p>I have searched the web and found a solution by Ripley but it does not work in my case:</p>
  
  <pre><code>levelplot(rcp852, xlab = "", ylab = "",
            panel = function(x, y, subscripts, ...) {
              panel.levelplot(x, y, subscripts, ...)
              panel.xyplot(topo$x,topo$y, cex = 0.5, col = 1)
            }
  )
  </code></pre>
  
  <p>I tried many other options but the points do not align with the map generated via <code>levelplot</code>.</p>