title: Grid lines disappear when using ggarrange with common.legend = TRUE
body: <p>I have two plots I'm trying to present together using ggarrange, but y grid lines disappear when doing so using common.legend = TRUE.</p>
  <p>Here's how it looks <strong>without</strong> common.legend = TRUE:</p>
  <pre><code>ggarrange(p1,p2)
  </code></pre>
  <p><a href="https://i.stack.imgur.com/plcuE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/plcuE.png" alt="enter image description here" /></a></p>
  <p>Setting a common legend results in this:</p>
  <pre><code>ggarrange(p1,p2,common.legend = TRUE)
  </code></pre>
  <p><a href="https://i.stack.imgur.com/VZVbK.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/VZVbK.png" alt="enter image description here" /></a></p>
  <p>Any ideas why this is happening and how to avoid it?</p>