title: Adjusting x limits xlim() in ggplot2 geom_density() to mimic ggvis layer_densities() behavior
body: <p>Is there a way to make ggplot2's <code>geom_density()</code> function mimic the behavior of ggvis's <code>layer_densities()</code>?  That is, make it so <code>p1</code> looks like <code>p3</code> (see below) without the call to <code>xlim()</code>?  Specifically, I prefer the view that smooths the tails of the density curve.</p>
  
  <pre><code>library(ggvis)
  library(ggplot2)
  
  faithful %&gt;% 
    ggvis(~waiting) %&gt;% 
    layer_densities(fill := "green") -&gt; p1
  
  ggplot(faithful, aes(x = waiting)) +
    geom_density(fill = "green", alpha = 0.2) -&gt; p2
  
  ggplot(faithful, aes(x = waiting)) +
    geom_density(fill = "green", alpha = 0.2) +
    xlim(c(30, 110)) -&gt; p3
  
  p1
  p2
  p3
  </code></pre>
  
  <p>ggvis Output:
  <img src="https://i.stack.imgur.com/szNZw.jpg" alt="p1"></p>
  
  <p>ggplot2 "default":
  <img src="https://i.stack.imgur.com/L39QX.png" alt="p2"></p>
  
  <p>ggplot2 "desired":
  <img src="https://i.stack.imgur.com/QN2U3.png" alt="p3"></p>
  
  <p>Note: One can make ggvis mimic ggplot2 via the following (using <code>trim=TRUE</code>), but I would like to go the other direction...</p>
  
  <pre><code>faithful %&gt;% 
    compute_density(~waiting, trim=TRUE) %&gt;% 
    ggvis(~pred_, ~resp_) %&gt;% 
    layer_lines()
  </code></pre>