title: Combining an sp object with ggmap
body: <p>I have read in sp object using <code>geo &lt;- readOGR</code>. And when I use <code>plot(geo)</code> I get the following image: </p>
  
  <p><a href="https://i.stack.imgur.com/0wfJV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0wfJV.png" alt="enter image description here"></a></p>
  
  <p>It is some data that I have downloaded from a public agency in Denmark. </p>
  
  <p>How can I combine the borders of the "states" from the above plot with the plot with ggmap bellow: </p>
  
  <pre><code>library(ggplot)
  library(maps)
  library(ggmap)
  
  
  ggplot() + borders(regions = "Denmark", colour = "gray50", fill = "gray50") 
  </code></pre>
  
  <p>What I would like to a achieve is a plot where each polygon has a color on a color scale based on some calculation. </p>