title: ggplot2 legend for stat_summary
body: <p>How can I create a legend informing that the red cross is the mean?</p>
  
  <pre><code>ggplot(results, aes(x=factor, y=proportionPositive)) +
  geom_boxplot() +
  stat_summary(fun.data = "mean_cl_normal", colour = "red", shape=4)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/4TJ7m.png" alt="enter image description here"></p>