title: How do you combine a map with complex display of points in ggplot2?
body: <p>I'm trying to plot points from study sites with a background map of Africa.  I can create the two independently, but I am having a hard time overlaying them on top of eachother.</p>
  
  <p>The map of Africa I am using is an Esri shapefile from maplibrary.org.  It is available from my dropbox at <a href="https://www.dropbox.com/s/etqdw3nky52czv4/Africa%20map.zip" rel="noreferrer">https://www.dropbox.com/s/etqdw3nky52czv4/Africa%20map.zip</a>.  I have the points in a text file, also available from my drop box.  <a href="https://www.dropbox.com/s/scvymytjsr5pvaf/SPM-437-22Nov12.txt" rel="noreferrer">https://www.dropbox.com/s/scvymytjsr5pvaf/SPM-437-22Nov12.txt</a>.  They refer to studies on molecular drug resistance of malaria parasites.  I would like to plot them so that the color is the proportion of parasites with the drug resistant genetic marker and the size is the number of parasites tested.  </p>
  
  <p>Plotting the points independently:</p>
  
  <pre><code>qplot(Longitude, Latitude, data = d.spm.437, colour = Frc437, size = Tot437)
  </code></pre>
  
  <p>Plotting the map of Africa:</p>
  
  <pre><code>library(maptools)
  africa = readShapePoly("Africa.shp")
  africa.map = fortify(africa, region="COUNTRY")
  qplot(long, lat, data = africa.map, geom="path", group=group)
  </code></pre>
  
  <p>Any help on putting these two together while preserving the display of the points would be appreciated.</p>