title: Add significance lines outside/between facets
body: <p>I wanted to add significant stars over 3 facets to compare them.</p>
  
  <p>I google online but it is so complicated to add things outside plot. There is a ggsignif package but it does nothing to facets (<a href="https://github.com/const-ae/ggsignif/issues/22" rel="nofollow noreferrer">https://github.com/const-ae/ggsignif/issues/22</a>). It seems possible using gridExtra but I cannot make it.</p>
  
  <p>The stars can be draw easily in a single plot, not facets. But I have to use facets to have separate rugs on the left. If you know how to have separate rugs inside a single plot, it should also solve the problem.</p>
  
  <p>Here is the code and plot I want to add things on:</p>
  
  <pre><code>library(ggplot2)
  ToothGrowth$dose = factor(ToothGrowth$dose)
  ggplot(ToothGrowth, aes(x='', y=len, color=dose)) +
    geom_boxplot() +
    geom_rug(sides="l") +
    facet_grid(. ~ dose)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/O03dY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/O03dY.png" alt="enter image description here"></a></p>
  
  <p>What I want is:</p>
  
  <p><a href="https://i.stack.imgur.com/pvgON.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/pvgON.png" alt="enter image description here"></a></p>
  
  <p>Sorry for the drawing. The line width should be the same. The final result should be really similar to this but for <strong>facets</strong>:</p>
  
  <p><a href="https://i.stack.imgur.com/ftxUS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ftxUS.png" alt="enter image description here"></a></p>