title: Filling countries and continents with maps library according to variable value
body: <p>I want to fill a map with some countries from UE, and then some continents like Asia and Africa, according to my variable <code>prob2</code> values . This is my data <code>map_d</code>:</p>
  
  <pre><code>state prob2
  &lt;chr&gt; &lt;dbl&gt;
  Germany   0.6
  Austria   2.9
  Belgium   1.9
  Bulgaria   0.6
  Cyprus   0.0
  Croatia   1.7
  ...
  Other Asian   9.2
  Other African   2.5
  Other North American  10.7
  Other Latin American   2.3
  Other Oceania   5.0
  </code></pre>
  
  <p>Firstly I fill the countries of Europe, using this code:</p>
  
  <pre><code>europ_map &lt;- map_data("world", region = c(
    "Germany", 
  "Austria",
  "Belgium",
  "Bulgaria",
  "Chipre",
  "Croacia",
  "Denmark",
  "Slovakia",
  "Slovenia",
  "Spain",
  "Estonia",
  "Finland",
  "France",
  "Greece",
  "Hungary",
  "Ireland",
  "Italy",
  "Latvia",
  "Lithuania",
  "Luxembourg",
  "Malta",
  "Norway",
  "Netherlands",
  "Poland",
  "Portugal",
  "UK",
  "Czech Republic",
  "Romania",
  "Sweden"))
  
  fin_map &lt;- merge(europ_map, map_d, by.x="region", by.y="state")
  library(plyr)
  fin_map &lt;- arrange(fin_map, group, order)
  
  ggplot(fin_map, aes(x=long, y=lat, group=group, fill=prob2)) +
    geom_polygon(colour = "white") +
    coord_map("polyconic")
  </code></pre>
  
  <p>Which produces this map:
  <a href="https://i.stack.imgur.com/JgCC6.jpg" rel="nofollow noreferrer">Europe Map</a></p>
  
  <p>Now, I need to add shape of continents to my map, and fill with the value of <code>prob2</code>. Is possible? </p>
  
  <p>I found in this post how to plot continents but is a different way to do: <a href="https://stackoverflow.com/questions/20146809/how-can-i-plot-a-continents-map-with-r">David Ameller's question</a>, and Im not able to add variable values throught this code.</p>
  
  <p>Thanks in advance!!</p>