title: ggplot map plot fails when limits set with coord_map
body: <p>I'm trying to limit a map plot to a specific area.  <code>coord_map</code> <a href="https://stackoverflow.com/questions/17644262/geographical-borders-incomplete-using-geom-polygon-for-plotting-map-ggplot2">is preferred</a> to <code>scale_x_continuous</code> and y equivalent, since the latter mess up the polygons.  But here I'm finding it fails for some reason.  Here's my code (downloads a 57kb shapefile):</p>
  
  <pre><code>require(maptools)
  require(ggplot2)
  download.file('https://dl.dropboxusercontent.com/u/46043231/UK.zip', "uk.zip", method="internal", mode="wb")
  unzip('uk.zip')
  uk = readShapePoly('uk_outline_1000m.shp')
  print(bbox(uk))
          min       max
  x  259.9625  655566.4
  y 7211.7025 1218558.9
  uk2 = fortify(uk)
  (p = ggplot(uk2, aes(x=long, y=lat, group=group)) + geom_polygon() + coord_equal())
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/zTQ24.png" alt="enter image description here"></p>
  
  <p>But when <code>coord_map</code> is used the plot disappears:</p>
  
  <pre><code>p + coord_map(xlim=c(0, 630000), ylim=c(0, 1000000))
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/jDQUY.png" alt="enter image description here"></p>
  
  <p>Any idea what's going on??</p>