title: Conditional fill in a density plot within same group
body: <p>I want to create a density plot with ridges where colour is conditional on crossing a certain threshold. </p>
  
  <p>The aim is to illustrate the distribution of earnings while highlighting those over a certain threshold (AUD $1368 per week).  I've provided a bare-bones plot of my attempt below.</p>
  
  <pre class="lang-r prettyprint-override"><code>ggplot(subset(vic, totalweekslost &gt; 0 &amp; nwe %in% 100:3000), 
         aes(nwe, fill = nwe &gt; 1368)) + 
    geom_density()
  </code></pre>
  
  <p>Instead of just changing the colour of those within a single group in the density plot, it creates a new group on its own scale.</p>
  
  <p><a href="https://i.stack.imgur.com/DHby9.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/DHby9.png" alt="reproduced here"></a></p>
  
  <p>What I want is for the colour change to be in the same group and a clear cut-point illustrated. I can see what is wrong with my code, but I can't figure out how to create what I'm after. </p>