title: How to save after overlaying a spatial point data frame onto raster
body: <p>I tried to overlay a <code>SpatialPointsDataFrame</code> (yellow birds) onto a <code>raster</code> data (map). This was successful with the following R codes: </p>
  
  <pre><code>map1 = plot(map)
  
  overlay_map = plot(yellowbirds,add=TRUE,col="blue")
  </code></pre>
  
  <p>However, when I want to regenerate the map by typing <code>overlay_map</code>, the output produced is <code>Null</code> instead of giving me the map.</p>
  
  <p>How do I save the above correctly as a variable, so that I can generate the map without typing out the code again?</p>