title: ggmap with geom_map superimposed
body: <pre><code>library(sp)
  library(spdep)
  library(ggplot2)
  library(ggmap)
  library(rgdal)
  </code></pre>
  
  <p>Get and fiddle with data:</p>
  
  <pre><code>nc.sids &lt;- readShapePoly(system.file("etc/shapes/sids.shp", package="spdep")[1],ID="FIPSNO", proj4string=CRS("+proj=longlat +ellps=clrk66"))
  nc.sids=spTransform(nc.sids,CRS("+init=epsg:4326"))
  </code></pre>
  
  <p>Get background map from stamen.com, plot, looks nice:</p>
  
  <pre><code>ncmap = get_map(location=as.vector(bbox(nc.sids)),source="stamen",maptype="toner",zoom=7)
  ggmap(ncmap)
  </code></pre>
  
  <p>Create a data frame with long,lat,Z, and plot over the map and a blank plot:</p>
  
  <pre><code>ncP = data.frame(coordinates(nc.sids),runif(nrow(nc.sids)))
  colnames(ncP)=c("long","lat","Z")
  
  ggmap(ncmap)+geom_point(aes(x=long,y=lat,col=Z),data=ncP)
  ggplot()+geom_point(aes(x=long,y=lat,col=Z),data=ncP)
  </code></pre>
  
  <p>give it some unique ids called 'id' and fortify (with vitamins and iron?)</p>
  
  <pre><code>nc.sids@data[,1]=1:nrow(nc.sids)
  names(nc.sids)[1]="id"
  ncFort = fortify(nc.sids)
  </code></pre>
  
  <p>Now, my map and my limits, I want to plot the 74 birth rate:</p>
  
  <pre><code>myMap = geom_map(aes(fill=BIR74,map_id=id),map=ncFort,data=nc.sids@data)
  Limits = expand_limits(x=ncFort$long,y=ncFort$lat)
  </code></pre>
  
  <p>and on a blank plot I can:</p>
  
  <pre><code>ggplot() + myMap + Limits
  </code></pre>
  
  <p>but on a ggmap I can't:</p>
  
  <pre><code>ggmap(ncmap) + myMap + Limits
  # Error in eval(expr, envir, enclos) : object 'lon' not found
  </code></pre>
  
  <p>Some versions:</p>
  
  <pre><code>&gt; packageDescription("ggplot2")$Version
  [1] "0.9.0"
  &gt; packageDescription("ggmap")$Version
  [1] "2.0"
  </code></pre>
  
  <p>I can add geom_polygon to ggplot or ggmap and it works as expected. So something is up with geom_map....</p>