title: How to adjust time scale axis for ggplot histogram
body: <p>I am working with a data frame where one of the columns consists of <code>POSIXct</code> date-time values.  I am trying to plot a histogram of these timestamps using <code>ggplot2</code> but I'm having two issues:</p>
  
  <ol>
  <li><p>I don't know how to set the binwidth in <code>geom_histogram()</code>.  I'd like to set each bin to a day or a week.  I've tried providing a difftime object, but I get an error.  I also tried <code>binwidth=1</code> but R just hangs.</p></li>
  <li><p>How do I set the limits in <code>scale_x_time()</code>?  The only way I could get it to work was by converting my <code>POSIXct</code> timestamps using <code>as.Date()</code>.</p></li>
  </ol>