title: Add a coloured border between facets in ggplot2
body: <p>I wish to add a thick dark grey bar in between each facet. I don't want a border around each facet, instead a dark grey bar in between plots.</p>
  
  <pre><code>library(ggplot2)
  
  ggplot(mpg, aes(cty, hwy, color = factor(year))) + 
    geom_point() +
    facet_wrap(~ cyl, nrow = 1) 
  </code></pre>
  
  <p>I found <a href="https://stackoverflow.com/questions/28652284/how-to-change-color-of-facet-borders-when-using-facet-grid">this</a> question although my data is as per the example above, only one row and not a <code>facet_grid</code>.</p>