title: Adding text to multiple column graph (faceted) in ggplot2
body: <p>I am trying to add p-values to each boxplot pair in the graph shown below. I would like the p-values to be placed under each soil horizon label ('O', 'A' and 'B'). 
  <a href="https://i.stack.imgur.com/J0RDw.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/J0RDw.jpg" alt="enter image description here"></a></p>
  
  <p>My data looks like this:</p>
  
  <pre><code>&gt; head(kiwi_l)
  # A tibble: 6 x 6
  type         horizon    root_name  length diameter n_child
  &lt;chr&gt;        &lt;chr&gt;      &lt;chr&gt;       &lt;dbl&gt;    &lt;dbl&gt;   &lt;int&gt;
  1 Elevated CO2 A      R1_A_L_S4G 0.0752   0.0342       0
  2 Elevated CO2 A      R1_A_L_S4F 0.0987   0.0319       0
  3 Elevated CO2 A      R1_A_L_S4E 0.105    0.0209       0
  4 Elevated CO2 A      R1_A_L_S4D 0.0476   0.0127       0
  5 Elevated CO2 A      R1_A_L_S4C 0.110    0.0282       0
  6 Elevated CO2 A      R1_A_L_S4B 0.244    0.0168       0
  </code></pre>
  
  <p>While the code I used to generate the graph is:</p>
  
  <pre><code>l_horizon&lt;-ggplot(kiwi, aes(x=type, y=length, fill=type, palette='jco')) 
  + 
  geom_boxplot() +
  facet_grid(. ~ factor(horizon, level=level_order)) +
  theme_pubr() +
  scale_y_continuous(name='Primary root length (cm)') +
  scale_x_discrete(name='Treatment') +
  ggtitle('Soil horizon') + theme(plot.title = element_text(hjust = 0.5)) +
  theme(legend.position="none") +
  theme(plot.title = element_text(size = 10, face = "bold"),
      text = element_text(size = 10),
      axis.title = element_text(face="bold"),
      axis.text.x=element_text(size = 10),
      axis.text.y=element_text(size=10),
      axis.title.x = element_blank(),
      axis.title.y=element_text(size=10))
  
      l_horizon&lt;-l_horizon+scale_fill_locuszoom()
      l_horizon
  </code></pre>
  
  <p>Please help!</p>