title: Connecting across missing values with geom_line
body: <p>I'm trying to figure out if it's possible to connect across missing values using geom_line. For example, in the link below there are missing values at time 3 in facet F. I'd like a line to connect time 2 and 4 in that case. Is there a way to achieve this?</p>
  
  <p><a href="https://farm8.staticflickr.com/7061/6964089563_b150e0c2a6.jpg" rel="noreferrer">https://farm8.staticflickr.com/7061/6964089563_b150e0c2a6.jpg</a></p>
  
  <p><img src="https://farm8.staticflickr.com/7061/6964089563_b150e0c2a6.jpg" alt=""></p>
  
  <p>I have a data frame of cumulative values like so:</p>
  
  <pre><code>head(cumulative)
  
    individual series Time     Value
  1          A      x    1 -1.008821
  2          A      x    2 -2.273712
  3          A      x    3 -3.430610
  4          A      x    4 -4.618860
  5          A      x    5 -4.893075
  6          A      x    6 -5.836532
  </code></pre>
  
  <p>Which I'm plotting with:</p>
  
  <pre><code>ggplot(cumulative, aes(x=Time,y=Value, shape=series)) + 
      geom_point() + 
      geom_line(aes(linetype=series)) + 
      facet_wrap(~ individual, ncol=3)
  </code></pre>