title: extract hours and seconds from POSIXct for plotting purposes in R
body: <p>Suppose I have the following <code>data.frame</code> <code>foo</code></p>
  
  <pre><code>           start.time duration
  1 2012-02-06 15:47:00      1
  2 2012-02-06 15:02:00      2
  3 2012-02-22 10:08:00      3
  4 2012-02-22 09:32:00      4
  5 2012-03-21 13:47:00      5
  </code></pre>
  
  <p>And <code>class(foo$start.time)</code> returns</p>
  
  <pre><code>[1] "POSIXct" "POSIXt" 
  </code></pre>
  
  <p>I'd like to create a plot of <code>foo$duration</code> v. <code>foo$start.time</code>.  In my scenario, I'm only interested in the time of day rather than the actual day of the year.  How does one go about extracting the time of day as hours:seconds from <code>POSIXct</code> class of vector?</p>