title: Combine multiple maps in R using ggplot2
body: <p>This is a reproductible example of an issue I am facing. I am trying to create maps with ggplot2 in multiple stages. Here is the issue I face. 
  Consider the data <code>border</code>with the polygons of states in US at the Mexican border, and <code>border.county</code>with the polygons of the counties in these states. The following code allows you to get the data: </p>
  
  <pre><code>library(maps)
  library(ggmap)
  library(ggplot2)
  
  USA &lt;- get_googlemap(center = 'usa', zoom = 4,  
                       style = 'administrative|element:labels|visibility:off')
  
  us.df &lt;- map_data("state") 
  border &lt;- subset(us.df, 
                   region %in% c("california","arizona","new mexico","texas"))
  
  counties &lt;- map_data("county")
  border.county &lt;- subset(counties,
                          region %in% c("california","arizona","new mexico","texas"))
  </code></pre>
  
  <p>Now I wan to create a map, with the background of a map from Google Maps, with the state polygons and the county borders. If I do the following, it works neatly: </p>
  
  <pre><code>Allmap &lt;- ggmap(USA) + 
    geom_polygon(aes(x = long, y = lat, fill = region, group = group), 
                 data=border,  color = "white") +
    geom_polygon(aes(x = long, y = lat,  group = group), 
                 data=border.county, fill=NA, color="red") 
  </code></pre>
  
  <p>Now if I wanted to create this map in multiple stages, I hit problems. I just want the county boundaries for  background information (as sort of 'recurrent theme'), and I will create multiple maps with changing information at the state level. So I create the 'background map' with counties, which works fine: </p>
  
  <pre><code>Countmap &lt;- ggmap(USA) + 
    geom_polygon(aes(x = long, y = lat,  group = group), 
                 data=border.county, fill=NA, color="red") 
  </code></pre>
  
  <p>And now I try to combine it with the state maps: </p>
  
  <pre><code>Statmap &lt;- ggmap(USA) + 
    geom_polygon(aes(x = long, y = lat, fill = region, group = group), 
                 data=border,  color = "white") + 
    Countmap
  </code></pre>
  
  <p>That gives me the error: </p>
  
  <pre><code> Error: Don't know how to add o to a plot
  </code></pre>
  
  <p>How can I solve this? I can combine the maps in the other way (as in: <code>Statmap &lt;- Countmap + geom_polygon(aes(x = long, y = lat, fill = region, group = group), data=border,  color = "white")</code>); however, that puts the counties under the state boundaries. </p>
  
  <p>I also know this specific problem has the easy solution of just drawing a map with the states first, and combine it with the counties in a second stage. However, in my real scenario, that is not an option because the recurrent theme of the map is something that needs to be drawn in second place : cities and geographic borders (like my county boundaries here). </p>
  
  <p>This is the map I want to create: </p>
  
  <p><a href="https://i.stack.imgur.com/HPbFT.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HPbFT.jpg" alt="enter image description here"></a> </p>