title: ggplot Donut chart
body: <p>Hi I really have googled this a lot without any joy. Would be happy to get a reference to a website if it exists. I'm struggling to understand the <a href="http://docs.ggplot2.org/current/coord_polar.html" rel="noreferrer">Hadley documentation on polar coordinates</a> and I know that pie/donut charts are considered inherently evil. </p>
  
  <p>That said, what I'm trying to do is </p>
  
  <ol>
  <li>Create a donut/ring chart (so a pie with an empty middle) like the <a href="https://tex.stackexchange.com/questions/17898/how-can-i-produce-a-ring-or-wheel-chart-like-that-on-page-88-of-the-pgf-manu">tikz ring chart</a> shown here</li>
  <li>Add a second layer circle on top (with <code>alpha=0.5</code> or so) that shows a second (comparable) variable.</li>
  </ol>
  
  <p>Why? I'm looking to show financial information. The first ring is costs (broken down) and the second is total income. The idea is then to add <code>+ facet=period</code> for each review period to show the trend in both revenues and expenses and the growth in both.</p>
  
  <p>Any thoughts would be most appreciated</p>
  
  <p>Note: Completely arbitrarily if an MWE is needed if this was tried with</p>
  
  <pre><code>donut_data=iris[,2:4]
  revenue_data=iris[,1]
  facet=iris$Species
  </code></pre>
  
  <p>That would be similar to what I'm trying to do.. Thanks</p>