title: Plotting google map with ggplot in R
body: <p>I am trying to plot <a href="http://maps.googleapis.com/maps/api/staticmap?center=Montegomery,AL&amp;zoom=11&amp;size=512x512&amp;scale=2&amp;maptype=roadmap&amp;markers=color:blue%7Clabel:S%7C40.702147,-74.015794&amp;markers=color:green%7Clabel:G%7C40.711614,-74.012318&amp;markers=color:red%7Ccolor:red%7Clabel:C%7C40.718217,-73.998284&amp;sensor=false" rel="noreferrer">Google map</a> that is queried using RgoogleMaps package and combine it with ggplot. Ultimately, I want to show total population using <code>geom_point</code>, somewhat similar to the picture below however I am trying to concentrate on Montgomery region because of over-plotting. </p>
  
  <p>I am frustrated because I cannot plot my queried map in R. I tried a couple of packages such as <code>read.jpeg</code> and <code>png</code> but it didn't quite work out.</p>
  
  <p>R codes:</p>
  
  <pre><code>#query google map
  al1 &lt;- GetMap(center=c(32.362563,-86.304474), zoom=11, destfile = "al2.jpeg", 
         format="jpg",maptype="roadmap")
  
  #load only specific states
  alabama &lt;- subset(all_states, region %in% c("alabama"))
  
  #population
  p1 &lt;- ggplot()
  p1 &lt;- p1 + geom_polygon(data=alabama, 
        aes(x=long, y=lat, group=group), colour="white", fill="grey10")
  p1 &lt;- p1 + geom_point(data=quote, aes(x=IntPtLon, y=IntPtLat, size=TotPop, 
        color=TotPop),colour="coral1") + scale_size(name="Total Pop")
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/m9k40.jpg" alt="enter image description here"></p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>Here is my rough result. I still want to:</p>
  
  <ul>
  <li>Change the scale of dots' size because they seem rather small on the map.</li>
  <li>Make dots transparent or not-filled so that the map is still visible.</li>
  </ul>
  
  <p><img src="https://i.stack.imgur.com/3zarN.png" alt="enter image description here"></p>
  
  <pre><code>al1 &lt;- get_map(location = c(lon = -86.304474, lat = 32.362563), zoom = 11, maptype = 'terrain')
  al1MAP &lt;- ggmap(al1)+ geom_point(data=quote_mgm, aes(x=IntPtLon, y=IntPtLat, size=TotPop))
  </code></pre>