title: Force X axis on both graphs in a facet grid when X values are the same
body: <p>I have data with about 30 categories for the X axis in two groups for faceting. I will show this with some random data:</p>
  
  <pre><code>dataf &lt;- data.frame(x=c(1:30), A=rnorm(30,20,5), B=rnorm(30,15,0.5))
  datam &lt;- melt(dataf, id="x")
  ggplot(datam, aes(factor(x), value)) + 
    geom_bar(stat="identity") + 
    facet_grid(variable ~ .)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/PM8ey.png" alt="enter image description here"></p>
  
  <p>This is just lovely, except that it would be easier to quickly read off categories on the top grouping if the x axis was reproduced on that graph too. However</p>
  
  <pre><code>ggplot(datam, aes(factor(x), value)) + 
    geom_bar(stat="identity") + 
    facet_grid(variable ~ ., scales="free")
  </code></pre>
  
  <p>makes no difference to the x axis because, I guess, the values are the same for both groupings.</p>
  
  <p>How can I force the X axis to be reproduced for the top group as well of bars?</p>