title: How to add text to geom_vline in ggplot
body: <p>I would like to add text to the top of the vertical line I add to my density plot in ggplot; something like "Target Length"; doesn't seem to be a solution for this anywhere.</p>
  
  <pre><code>ggplot(iris, aes(x = Sepal.Length,  fill = Species)) + 
    geom_density(alpha = 0.5) +
    geom_vline(xintercept = 6, linetype="dashed", 
               color = "black", size=1, )
  </code></pre>
  
  <p>I would prefer to have a box like possible in geom_label but some variation of geom_text will also be sufficient.</p>