title: Pie charts in ggplot2 with variable pie sizes
body: <p>I've tried various ways to get a <code>facet_grid</code> of pie charts in ggplot2 to vary width/radii according to another variable (strength).</p>
  
  <p>geom_bar accepts width=0.5 as a parameter but it is ignored once <code>coord_polar</code> is added.  <code>Adding width=0.5</code> to the ggplot <code>aes</code> or adding a <code>aes</code> to <code>geom_bar</code> doesn't work.  I can't see any other relevant options for <code>coord_polar</code>.  What's the easiest way to do this? The code below makes a nice grid of pie charts but doesn't change the sizes of the pie charts.  What am I missing?</p>
  
  <pre><code>mydata &lt;- data.frame(side1=rep(LETTERS[1:3],3,each=9),side2=rep(LETTERS[1:3],9,each=3),widget=rep(c("X","Y","Z"),9*3),val=runif(9*3),strength=rep(c(1,2,3),3,each=3))
  ggplot(mydata, aes(x="",y = val, fill = widget, width = strength)) +
  geom_bar(position="fill") + facet_grid(side1 ~ side2) +
  coord_polar("y") + opts(axis.text.x = theme_blank()) 
  </code></pre>