title: Add custom lines in ggplot barplot
body: <p>I created this plot
  <img src="https://i.stack.imgur.com/HVZRy.jpg" alt="enter image description here"></p>
  
  <p>with this code:</p>
  
  <pre><code>ggplot(data_long, aes(Trial,value,fill=factor(Trial))) +    
      stat_summary(fun.y=mean,geom="bar") + facet_grid(Task~Gruppo) + labs(x="Trial 
      type",y="Accuracy %") + theme(legend.position="none")
  </code></pre>
  
  <p>Now, I need to add custom lines that show the differences between couples of values. Here is an example of what I want to do (see the first two bars with p = 0.46):</p>
  
  <p><img src="https://i.stack.imgur.com/h7DHJ.jpg" alt="enter image description here"></p>
  
  <p>I have no idea about solution, and the things are also more complicated for me since I used <code>facet_grid</code>.
  Can anyone help me?</p>