title: How do you add a general label to facets in ggplot2?
body: <p>I often have numeric values for faceting.
  I wish to provide sufficient information to interpret these faceting values in a supplemental title, similar to the axis titles.
  The labeller options repeat much unnecessary text and are unusable for longer variable titles.</p>
  
  <p>Any suggestions?</p>
  
  <p>The default:</p>
  
  <pre><code>test&lt;-data.frame(x=1:20, y=21:40, facet.a=rep(c(1,2),10), facet.b=rep(c(1,2), each=20))
  qplot(data=test, x=x, y=y, facets=facet.b~facet.a)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/2VZA3.jpg" alt="enter image description here"></p>
  
  <p>What I would love:</p>
  
  <p><img src="https://i.stack.imgur.com/y11DK.jpg" alt="enter image description here"></p>
  
  <p>The best I can do in ggplot:</p>
  
  <pre><code>qplot(data=test, x=x, y=y)+facet_grid(facet.b~facet.a, labeller=label_both)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/OJvhj.jpg" alt="enter image description here"></p>
  
  <p>As indicated by @Hendy, similar to:
  <a href="https://stackoverflow.com/questions/9096671/add-a-secondary-y-axis-to-ggplot2-plots-make-it-perfect">add a secondary y axis to ggplot2 plots - make it perfect</a></p>