title: Add colour based distinction to R bubble charts
body: <p>I'd come across a bubble chart tutorial <a href="http://www.matthewmaenner.com/blog/2010/11/23/how-to-make-bubble-charts-in-ggplot2/" rel="nofollow">here</a>, whose code I'm reusing for clarifications. Please find the code below:</p>
  
  <pre><code>crime &lt;-read.csv("http://datasets.flowingdata.com/crimeRatesByState2005.tsv", header=TRUE, sep="\t")
  
  popIndex &lt;- crime$population/max(crime$population)
  crime.new &lt;- cbind(crime,popIndex)
  
  ggplot(crime, aes(x=murder, y=burglary, size=population, label=state),guide=FALSE)+
  geom_point(colour="white", fill="#E69F00", shape=21)+ scale_size_area(max_size = 22)+
  scale_x_continuous(name="Murders per 1,000 population", limits=c(0,12))+
  scale_y_continuous(name="Burglaries per 1,000 population", limits=c(0,1250))+
  geom_text(size=4)+
  theme_bw()
  </code></pre>
  
  <p>In this plot, the bubble size is determined by the population. My question is, if I've to accommodate the <code>popIndex</code> variable in the plot using bubble colour, how do I go about it ?</p>