title: stacked bars within grouped bar chart
body: <p>I have the following graph</p>
  
  <pre><code>test &lt;- expand.grid('cat' = LETTERS[1:5], 'cond'= c(F,T), 'year' = 2001:2005)
  test$value &lt;- floor((rnorm(nrow(test)))*100)
  test$value[test$value &lt; 0] &lt;- 0
  
  ggplot() +
    geom_bar(data=test, aes(y = value, x = year, fill = cat), stat="identity",position='dodge') +
    theme_bw()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/pfs5p.png" alt="test">
  and I need to divide each 'cat' by 'cond'(true or false). How do I do that?</p>