title: Multiple titles in facet_wrap (ggplot2)
body: <p>I am trying to add multiple titles to a plot using facet_wrap and ggplot2. Say that you e.g. have quarterly data over two years, and wants a graphical comparison of the quarterly data with two <em>major</em> titles; 2014 and 2015, as well as titles for the respective quarter. </p>
  
  <p>I have come this far: </p>
  
  <pre><code>data &lt;- rnorm(10)
  
  A1 &lt;- data.frame("Y"=data, "X"=1:10, "Q"=1, "year"=2014)
  A2 &lt;- data.frame("Y"=data, "X"=1:10, "Q"=2, "year"=2014)
  A3 &lt;- data.frame("Y"=data, "X"=1:10, "Q"=3, "year"=2014)
  A4 &lt;- data.frame("Y"=data, "X"=1:10, "Q"=4, "year"=2014)
  
  N1 &lt;- data.frame("Y"=data, "X"=1:10, "Q"=1, "year"=2015)
  N2 &lt;- data.frame("Y"=data, "X"=1:10, "Q"=2, "year"=2015)
  N3 &lt;- data.frame("Y"=data, "X"=1:10, "Q"=3, "year"=2015)
  N4 &lt;- data.frame("Y"=data, "X"=1:10, "Q"=4, "year"=2015)
  
  A &lt;- rbind(A1, A2, A3, A4)
  N &lt;- rbind(N1, N2, N3, N4)
  tmp &lt;- data.frame(rbind(A, N))
  
  ggplot(data=tmp, aes(x=X, y=Y)) + geom_line() + facet_wrap(~year + Q, scales="free", ncol=4)
  </code></pre>
  
  <p>which gives me this graph:
  <a href="https://i.stack.imgur.com/l7APV.png" rel="noreferrer"><img src="https://i.stack.imgur.com/l7APV.png" alt="enter image description here"></a></p>
  
  <p>Instead I would like "2014" and "2015" to be in two separate grey boxes above each sub graph. Is that possible?</p>
  
  <p>Thanks!</p>