title: Adding "all" category to ggplots2 plot
body: <p><strong>Question</strong></p>
  
  <p>Suppose I want to make density plots for each categories with an additional category of "all" points. How would I go about creating such a figure? Is there a way other than duplicating all points with an "all" category?</p>
  
  <p><em>Example</em></p>
  
  <pre><code>library('ggplot2')
  fig &lt;- ggplot(iris, aes(x = Sepal.Length, color = Species)) + 
         stat_density(geom = "line", position = "identity")
  </code></pre>
  
  <p>which will output the following figure: </p>
  
  <p><a href="https://i.stack.imgur.com/Fm0T4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Fm0T4.png" alt="http://127.0.0.1:16553/chunk_output/8D7C71B83681FBD4/144E3B0/cby49jvbfsi3m/00001b.png"></a></p>
  
  <p>However, I would like to include another line that contains all of the points.</p>