title: How do I plot only the time portion of a timestamp including a date?
body: <p>So I have a set of timestamps like this:</p>
  
  <pre><code>datetime&lt;-c("2011-09-28 03:33:00", "2011-08-24 13:41:00", "2011-09-19 16:14:00", "2011-08-18 11:01:00", "2011-09-17 06:35:00", "2011-08-15 12:48:00")
  </code></pre>
  
  <p>I want to make a histogram of only the times. What I did was to split the column at the space to get only the times, then convert back to a POSIXct object in order for qplot to plot it:</p>
  
  <pre><code>library(ggplot2, stringr)    
  qplot(as.POSIXct(strptime((str_split_fixed(as.character(time), " ", 2)[,2]), "%H:%M:%S")))
  </code></pre>
  
  <p>However, the output of <code>as.POSIXct(strptime((str_split_fixed(as.character(datetime), " ", 2)[,2]), "%H:%M:%S"))</code> is</p>
  
  <pre><code>"2011-10-04 03:33:00 PDT" "2011-10-04 13:41:00 PDT" "2011-10-04 16:14:00 PDT" "2011-10-04 11:01:00 PDT" "2011-10-04 06:35:00 PDT" "2011-10-04 12:48:00 PDT"
  </code></pre>
  
  <p>qplot plots what I want, but this seems like a convoluted hack to me. Surely there's a better way to do this? I could convert into epoch time and plot that, but I was trying to avoid having to do that as an extra step.</p>
  
  <p>The larger question is, "How do I control the output of strptime?"</p>