title: Example Needed: Using arrow() with ggplot2
body: <p>I'd like to create a geom_path() that has arrows pointing towards the next location in the path. </p>
  
  <p>I can get the path to plot, without issue, for example: </p>
  
  <pre><code>df &lt;- (x=1:12, y=20:31, z=1:12)
  p &lt;- ggplot(df, aes(x=x, y=y))
  p + geom_point() + geom_path()
  </code></pre>
  
  <p>Now what I'd like to be able to do is plot that point arrows from one element in the path to the next. </p>
  
  <p>Extra marks if you could tell me how to smooth the lines from one element in the path to the next. </p>