title: ggplot2 histogram/bar chart with date and time
body: <p>Given this <code>df</code>:</p>
  
  <pre><code>       Date     Time     Values 
  1 2018-04-02 09:15:00 14.00000
  2 2018-04-02 09:30:00  4.00000
  3 2018-04-02 09:45:00 20.00000
  4 2018-04-02 10:00:00 30.00000
  5 2018-04-02 10:15:00 14.00000
  6 2018-04-02 10:30:00 31.16667
  </code></pre>
  
  <p>I'm trying to do a histogram/bar chart ordered by <code>Values</code>, while the X axis values are <code>Date</code> or <code>Time</code> (two different charts). This is the example with <code>Time</code> chart:</p>
  
  <pre><code>ggplot(df, aes(reorder(Time, Values)), y=Values) + 
    geom_col(position = "dodge")
  </code></pre>
  
  <p>I have been trying with <code>geom_col</code> and <code>geom_bar</code> with stat="identity" and other cases, but nothing seems to work for both charts. For the code above it appears this error:</p>
  
  <pre><code>Error in as.Date.numeric(value) : 'origin' must be supplied
  </code></pre>