title: Breaks are not displayed for scale_gradient when drawing a map
body: <p>I have a quite simple question I suppose. If I draw a scatter plot like this: </p>
  
  <pre><code> data &lt;- data.frame(lon=runif(100)*3+10, lat=runif(100)*5+42, value=rnorm(100)*100)
   ggplot() + geom_point(aes(x=lon, y=lat,  color=value), data=data)
  </code></pre>
  
  <p>the legend is properly displayed and the breaks are specified. </p>
  
  <p><img src="https://i.stack.imgur.com/j2mP1.jpg" alt="propper legend"></p>
  
  <p>but when i use a map as background:</p>
  
  <pre><code> SHmap &lt;- qmap(c(lon=mean(data[,'lon'], na.rm=TRUE), lat=mean(data$lat, na.rm=TRUE)), zoom=7)
   SHmap &lt;- SHmap + geom_point(aes(x=lon, y=lat,  color=value), data=data)
   SHmap
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/5GPba.jpg" alt="no breaks specified here"></p>
  
  <p>am I doing something wrong or is this behaviour intended?</p>