title: ggplot2: NA values not disconnecting lines when stacking them
body: <p>Given </p>
  
  <pre><code>library(ggplot2)
  df &lt;- data.frame(x=rep(1:5, 2), y=1:10, f=gl(2, 5, labels = letters[1:2])) 
  df$y[df$x==3] &lt;- NA
  df
  #    x  y f
  # 1  1  1 a
  # 2  2  2 a
  # 3  3 NA a
  # 4  4  4 a
  # 5  5  5 a
  # 6  1  6 b
  # 7  2  7 b
  # 8  3 NA b
  # 9  4  9 b
  # 10 5 10 b
  </code></pre>
  
  <p>I get this figure using ggplot2's default <code>position_identity</code> in <code>geom_line</code>:</p>
  
  <pre><code>ggplot(df, aes(x, y, color=f)) + 
    geom_line() + 
    geom_point(size=5)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/Ip7ed.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Ip7ed.gif" alt="enter image description here"></a></p>
  
  <p>And I get this figure when using <code>position_stack</code>: </p>
  
  <pre><code>ggplot(df, aes(x, y, color=f)) + 
    geom_line(position="stack") + 
    geom_point(size=5, position="stack")
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/VC8L3.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/VC8L3.gif" alt="enter image description here"></a></p>
  
  <p>Question: How do I indicate the <code>NA</code> values when using <code>position_stack</code>, i.e. how do I get the "hole" in the 2nd example? </p>
  
  <p>My <code>packageVersion("ggplot2")</code> is <code>2.1.0</code>. Thanks in advance. </p>