title: Add labels to each subplots created using facet_wrap (ggplot2)
body: <p>I am quite new to ggplot2 (and R) so bear with me. Using <code>facet_wrap</code>, I have created a multi-panel plot. I want to label them with alphabetical letters (starting with the top-left panel labelled with (A), and the next panel with (b) and so on.) but I am not sure how to go about doing this. I want this to be located outside of the plot (ideally at the top left of each panel).</p>
  
  <p>I came across <code>grid.text</code> and <code>ggtitle</code>, but I am not sure how I would implement these to get the desired outcome.</p>
  
  <p>I came across <a href="https://stackoverflow.com/questions/15690626/cant-label-multi-panel-figure-in-r-while-using-ggplot">this post</a> and with the given answers it seems possible to manually label each figure, but I would like the labelling to be done automatically, in order.</p>
  
  <p>I would deeply appreciate any advice/guides.</p>