title: How to create a county map with select counties highlighted?
body: <p>I'm trying to highlight select counties on a map of North Dekoda. How would I go about doing this? I have gotten this far so far. </p>
  
  <pre><code>library(ggplot2)
  library(maps)
  
  county_df &lt;- map_data('county')                          # mappings of counties by state
  countyMap &lt;- subset(county_df, region=="north dakota")   # subset just for NYS
  countyMap$county &lt;- countyMap$subregion
  
  cnames &lt;- aggregate(cbind(long, lat) ~ subregion, data=countyMap, FUN=mean)
  
  ggplot(countyMap, aes(long, lat)) +  
    geom_polygon(aes(group=group), colour='black', fill=NA) +
    geom_text(data=cnames, aes(long, lat, label = subregion), size=3) +
    coord_map()
  </code></pre>
  
  <p>I would like to highlight the following counties.</p>
  
  <pre><code>countynames &lt;- c("Adams",   "Billings", "Bottineau", "Bowman", "Burke",
                   "Divide", "Dunn", "Golden Valley", "Hettinger", "McHenry",
                   "McKenzie", "McLean", "Mercer", "Mountrail", "Renville", 
                   "Slope", "Stark", "Ward", "Williams")
  </code></pre>
  
  <p>Would this be possible?</p>