title: Plot ratio of geom_bar on second y-axis
body: <p>I would like to plot the ratio of geom_bar using a plot_line on a second axis. 
  Here is my dataframe :</p>
  
  <pre><code>df &lt;- data.frame(code=c('F6', 'F6','D4', 'D4', 'F5', 'F5', 'C4', 'C4', 'F7', 'F7'),
             group=c('0','1','0','1','0','1','0','1','0','1'),
             count=c(80, 700, 30, 680, 100, 360, 70, 230, 40, 200))
  </code></pre>
  
  <p>For the moment, I plot the following figure :</p>
  
  <pre><code>ggplot(df, aes(x=code, y=count, fill=group)) +
    geom_bar(stat ="identity", position="dodge")
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/tSgdV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/tSgdV.png" alt="enter image description here"></a></p>
  
  <p>And I would like to have also the ratio between groups.
  For example, for C4 it would be 70/230*100=30%. Here is what it could represent:</p>
  
  <p><a href="https://i.stack.imgur.com/OONa8.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/OONa8.png" alt="enter image description here"></a></p>
  
  <p>Any idea ?</p>