title: How to make a bar graph in ggplot that groups months of different years
body: <p>My dataframe, <code>df</code>:</p>
  
  <pre><code>df
     EffYr EffMo count        dts
  2   2012     1     1 2012-01-01
  3   2012     2     3 2012-02-01
  4   2012     3     1 2012-03-01
  5   2012     5     1 2012-05-01
  6   2012     6     1 2012-06-01
  7   2012     7     2 2012-07-01
  8   2012     8    11 2012-08-01
  9   2012     9    84 2012-09-01
  10  2012    10   184 2012-10-01
  11  2012    11   165 2012-11-01
  12  2012    12   246 2012-12-01
  13  2013     1   414 2013-01-01
  14  2013     2   130 2013-02-01
  15  2013     3   182 2013-03-01
  16  2013     4   261 2013-04-01
  17  2013     5   229 2013-05-01
  18  2013     6   249 2013-06-01
  19  2013     7   330 2013-07-01
  20  2013     8   135 2013-08-01
  </code></pre>
  
  <p>Each row of <code>df</code> represents a "month-year", the earliest being Jan 2012 and the latest being Aug 2013.  I want to plot a bar graph (using ggplot2) where each bar represents a row of <code>df</code> with the bar height equal to the row's <code>count</code>.  So, I should have 24 bars in total.</p>
  
  <p>I want my x axis to be divided into 12 intervals: Jan-Dec, and bars that represent the same calendar month should lie in the same "month interval". For example, if <code>df</code> has a row for Jan 2011, Jan 2012, Jan 2013, then the Jan portion of my graph should have 3 bars so that I can compare my business's performance in the month of January for subsequent years.</p>
  
  <p>Thanks</p>
  
  <p>Edit: I want something that looks like </p>
  
  <pre><code>ggplot(diamonds, aes(cut, fill=cut)) + geom_bar() +
    facet_grid(. ~ clarity)
  </code></pre>
  
  <p>But broken down by month.  I tried to modify that code to fit my data, but never could get it right.</p>