title: igraph add to geographic map
body: <p>I'm using R for networks. I've used the 'network' package, but now using 'igraph'.</p>
  
  <p>I can plot the igraph using geographic coords.
  I can plot a map using 'maptools'(readShapePoly).</p>
  
  <p>How can I plot the igraph on top of the map?</p>
  
  <p>I have tried new=FALSE and add=TRUE but it doesn't work, plotting the igraph object always overwrites.</p>
  
  <pre><code>greece &lt;- readShapePoly.("Z:/GeoData/World_data/Basemaps/Greece/GRC_adm1.shp")
  
  df&lt;-data.frame("from" = c.("Athens", "Iraklio", "Thessaloniki", "Patra"), "to"= c("Thessaloniki", "Thessaloniki", "Athens", "Iraklio"))
  meta &lt;- data.frame("name"=c("Athens", "Iraklio", "Thessaloniki", "Patra"), 
                 "lon"=c(23.72800,25.13356,22.94090,21.73507),  
                 "lat"=c(37.98415,35.33349,40.63229,38.24628))
  
  
  #plot it
  g &lt;- graph.data.frame(df, directed=T, vertices=meta)
  lo &lt;- layout.norm(as.matrix(meta[,2:3]))
  plot(greece)
  plot.igraph(g, layout=lo, add = T)
  </code></pre>
  
  <p>I've seen this solution, but I don't want to use the cario library as I'm using geographic data, not an image as base plot.
  <a href="https://stackoverflow.com/questions/30647537/using-geo-coordinates-as-vertex-coordinates-in-the-igraph-r-package/30671830#30671830">Using geo-coordinates as vertex coordinates in the igraph r-package</a></p>