title: Draw a "grid" between arranged plots
body: <p>I'm working with 4 different plots and I'm using ggarrange() from the ggpubr-package to put them in a single plot. I've prepared an example:</p>
  
  <pre><code>library(ggpubr)
  library(ggplot2)
  
  p1 &lt;- ggplot(iris, aes(x = Sepal.Length, y = Sepal.Width)) + geom_point() + ggtitle("Plot 1")
  p2 &lt;- ggplot(iris, aes(x = Petal.Length, y = Petal.Width)) + geom_point() + ggtitle("Plot 2")
  p3 &lt;- ggplot(iris, aes(x = Sepal.Length, y = Petal.Width)) + geom_point() + ggtitle("Plot 3")
  p4 &lt;- ggplot(iris, aes(x = Petal.Length, y = Sepal.Width)) + geom_point() + ggtitle("Plot 4") +
    facet_wrap(~Species)
  
  plot.list &lt;- list(p1, p2, p3, p4)
  
  ggarrange(plotlist = plot.list)
  </code></pre>
  
  <p>Output: 
  <a href="https://i.stack.imgur.com/CGai6.png" rel="noreferrer"><img src="https://i.stack.imgur.com/CGai6.png" alt="enter image description here"></a></p>
  
  <p>I would like to draw a border around the single plots, like so:</p>
  
  <p><a href="https://i.stack.imgur.com/nRevJ.png" rel="noreferrer"><img src="https://i.stack.imgur.com/nRevJ.png" alt="enter image description here"></a></p>
  
  <p>Is there any way to draw this border? Thanks!</p>