title: Centered X-axis label for muliplot using cowplot package
body: <p>I have a multiplot figure consisting of 4 plots in a 2x2 configuration.  I arranged the plots using the "cowplot" package and the plot_grid function using the code below</p>
  
  <pre><code>plot_grid(p1, p2, p3, p4, align='vh', vjust=1, scale = 1)
  </code></pre>
  
  <p>where p1-p4 are my 4 plots.  The resulting figure has an x-axis label associated with each column in the multiplot:</p>
  
  <p><a href="https://i.stack.imgur.com/O10js.png" rel="noreferrer"><img src="https://i.stack.imgur.com/O10js.png" alt="enter image description here"></a></p>
  
  <p>Does anyone know how I can code a single x-axis label centered at the bottom of the multiplot, either with cowplot or another way?</p>