title: Specifying the colour scale for maps in ggplot
body: <p>Having difficulty setting the colour scales for maps in ggplot. I need greyscale. Very grateful for an idea where I'm going wrong.  I also wonder if there is a more efficient way of getting the colour variable into ggplot (i.e. than by attaching it to 'fortified' data)?</p>
  
  <pre><code>library(ggplot2)
  states &lt;- map_data("state")
  var &lt;- data.frame(table(states$region)) # using rows as a dummy variable
  states$variable &lt;- var$Freq[match(states$region,var$Var1)]
  
  map &lt;- ggplot(states, aes(x=long, y=lat)) + 
    geom_polygon(aes(group=group, fill=variable), col=NA,lwd=0)
  
  map + scale_colour_gradient(low='white', high='grey20')
  map + scale_colour_grey()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/7Jpml.png" alt="enter image description here"></p>