title: Proportionally sized symbols in ggplot
body: <p>Using ggplot to plot proportional area symbols seems to require using <code>sqrt()</code> to achieve true proportionality:</p>
  
  <pre><code>require(ggplot2)
  
  t &lt;- data.frame(x=rep(c(1:5),5), y=rep(c(1:5),each=5), s=round(seq(1,100,length.out=25)))
  t
  p &lt;- ggplot(data=t, aes(x=x,y=y))
  
  # direct size-to-variable mapping
  p + geom_point(aes(size=s), pch=22, fill='#0000FF75', col=NA) +
    scale_size(range = c(1, 40)) +
    geom_text(data=t, aes(x=x,y=y,label=s),size=3,vjust=1)
  
  # proportional area size-to-variable mapping
  p + geom_point(aes(size=sqrt(s)), pch=22, fill='#0000FF75', col=NA) +
    scale_size(range = c(1, 40)) +
    geom_text(data=t, aes(x=x,y=y,label=s),size=3,vjust=1)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/jnfhV.png" alt="enter image description here"></p>
  
  <p>As you can see the labels are rooted when I need them to show the original data.  Have tried playing with the scale_size options but nothing works.  Anyone know a fix to this, or 
  is there maybe an obscure setting to achieve proportional area size mapping?</p>
  
  <p>Thanks in advance.</p>