title: Label individual panels in a multi-panel ggplot2
body: <p>I'm interested in trying to create simple corner labels for a multipanel figure I am preparing in ggplot. <a href="https://stackoverflow.com/questions/15867263/ggplot2-geom-text-with-facet-grid">This is similar to this previously asked question</a>, but the answers only explained how to include a label at the top of the plot, not produce a corner label in the format required by many journals. I hope to replicate something similar to the <code>plotrix</code> function <code>corner.label()</code> in <code>ggplot2</code>.</p>
  
  <p>Here is an example using <code>plottrix</code> of what I would like to recreate in <code>ggplot2</code>.</p>
  
  <pre><code>require(plotrix)
  
  foo1&lt;-rnorm(50,25,5)
  foo2&lt;-rpois(50,25)
  foo3&lt;-rbinom(50,25,0.5)
  foo4&lt;-rnbinom(50,25,0.5)
  
  par(mfrow=c(2,2))
  hist(foo1)
  corner.label(label='a',figcorner=T)
  hist(foo2)
  corner.label(label='b',figcorner=T)
  hist(foo3)
  corner.label(label='c',figcorner=T)
  hist(foo4)
  corner.label(label='d',figcorner=T)
  </code></pre>
  
  <p>This produces the following:</p>
  
  <p><img src="https://i.stack.imgur.com/6dfMW.png" alt="enter image description here"></p>
  
  <p>Thanks for any help in advance!</p>