title: Suppress NA column when faceting
body: <p>In my plot faceting occurs by two columns:</p>
  
  <pre><code>ggplot(peakDF, aes (Mass, Intensity)) + geom_line()+ facet_grid(file ~ fragment, scales = "free")
  </code></pre>
  
  <p>in the fragment column I have a lot of NA values so a column of plots is produced with these NAs as shown below:</p>
  
  <p><a href="https://i.stack.imgur.com/DArx0.png" rel="nofollow"><img src="https://i.stack.imgur.com/DArx0.png" alt="enter image description here"></a></p>
  
  <p>the problem is I don't want this column, but I can't simply filter on NA values in this column, because this would remove normalisation (across rows) which I really need. Any suggestions on how to preserve the whole dataset (for normalisation), yet not to plot the NA column?</p>