title: ggpubr stat_compare_means: Show significance levels with two grouping variables
body: <p>I am trying to visualize significance levels (asterisks) with <code>ggpubr</code>'s <code>stat_compare_means()</code>. I encountered the following issue: As opposed to <code>compare_means()</code>, you cannot add a grouping variable to the comparison.
  Example:</p>
  
  <pre><code>ggbarplot(ToothGrowth, x = "dose", y = "len", add = "mean_se", color = "supp",
  fill = "supp",position = position_dodge(0.8),add.params = list(group = "supp"))+
  stat_compare_means(ref.group = "0.5", group.by = "supp",label = "p.signif")
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/zAisS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zAisS.png" alt="plot"></a>
  As you can see, the significance levels are not displayed above all the bars, but only above the different doses, because ggpubr doesn't differentiate between different supps.</p>
  
  <p>Is there any way to compare to these (sub)sets as well?</p>
  
  <p>Thank you</p>