title: How to enforce stack ordering in ggplot geom_area
body: <p>Is it possible to enforce the stack order when using <code>geom_area()</code>? I cannot figure out why <code>geom_area(position = "stack")</code> produces this strange fluctuation in stack order around 1605.</p>
  
  <p>There are no missing values in the data frame.</p>
  
  <pre class="lang-r prettyprint-override"><code>library(ggplot2)
  
  counts &lt;- read.csv("https://gist.githubusercontent.com/mdlincoln/d5e1bf64a897ecb84fd6/raw/34c6d484e699e0c4676bb7b765b1b5d4022054af/counts.csv")
  
  ggplot(counts, aes(x = year, y = artists_strict, fill = factor(nationality))) + geom_area()
  </code></pre>
  
  <p><img src="https://i.imgur.com/EYy9Jbv.png" alt=""></p>