title: How to add mean, and mode to ggplot histogram?
body: <p>I need to add a mean line and the value of the mode for example to this kinds 
  of plots:</p>
  
  <p>I use this for calculate the number of bins:</p>
  
  <pre><code>bw &lt;- diff(range(cars$lenght)) / (2 * IQR(cars$lenght) / length(cars$lenght)^(1/3))
  </code></pre>
  
  <p>And the plot:</p>
  
  <pre><code>ggplot(data=cars, aes(cars$lenght)) + 
    geom_histogram(aes(y =..density..), 
                   col="red",
                   binwidth = bw,
                   fill="green", 
                   alpha=1) + 
    geom_density(col=4) + 
    labs(title='Lenght Plot', x='Lenght', y='Times')
  
  cars$lenght
  </code></pre>
  
  <p>168.8 168.8 171.2 176.6 176.6 177.3 192.7 192.7 192.7 178.2
    176.8 176.8 176.8 176.8 189.0 189.0 193.8 197.0 141.1 155.9
    158.8 157.3 157.3 157.3 157.3 157.3 157.3 157.3 174.6 173.2</p>
  
  <p>Thanks in advance.</p>