title: Arrange grouped local data frame in descending order
body: <p>I have a local data frame that I'm trying to group by 2 variables ("yr" and "mo"), get the mean of the data in each group and sort the results so the most recent data appears at the top in descending order.  However, I can't figure out how to get the "yr" variable to sort in descending order.  The "yr" variable is erroneously displayed in ascending order.</p>
  
  <pre><code>library(dplyr)
  df &lt;- tbl_df(data.frame(yr = c(2009, 2009, 2009, 2010, 2010, 2010, 2011, 2011, 2011), 
                      qtr = c(1, 1, 1, 1, 1, 2, 2, 2, 2),
                      mo = c(1, 1, 2, 3, 3, 4, 5, 5, 5), 
                      date = as.Date(c("2009-01-01", "2009-01-02","2009-02-01",
                                       "2010-03-01","2010-03-02","2010-04-01",
                                       "2011-05-01","2011-05-02","2011-05-03")),
                      x = c(10, 20, 30, 40, 50, 60, 70, 80, 90),
                      y = c(2, 4, 6, 8, 10, 12, 14, 16, 18),
                      z = c(1, 3, 5, 7, 9, 11, 13, 15, 17)))
  df %&gt;%
      select(yr, mo, x:z) %&gt;%
      group_by(yr, mo) %&gt;%
      summarize_each(funs(mean)) %&gt;%
      arrange(desc(yr), desc(mo))
  
  Source: local data frame [5 x 5]
  Groups: yr [3]
  
   yr    mo     x     y     z
  (dbl) (dbl) (dbl) (dbl) (dbl)
  1  2009     2    30     6     5
  2  2009     1    15     3     2
  3  2010     4    60    12    11
  4  2010     3    45     9     8
  5  2011     5    80    16    15
  </code></pre>
  
  <p>If I remove "desc(yr)" and just use "yr" in the arrange() function, I get the same results.  </p>
  
  <pre><code>df %&gt;%
        select(yr, mo, x:z) %&gt;%
        group_by(yr, mo) %&gt;%
        summarize_each(funs(mean)) %&gt;%
        arrange(yr, desc(mo))
  
  Source: local data frame [5 x 5]
  Groups: yr [3]
  
   yr    mo     x     y     z
  (dbl) (dbl) (dbl) (dbl) (dbl)
  1  2009     2    30     6     5
  2  2009     1    15     3     2
  3  2010     4    60    12    11
  4  2010     3    45     9     8
  5  2011     5    80    16    15
  </code></pre>
  
  <p>If I remove the "desc(mo)" and just use "mo" in the arrange function, I get the expected results and the data is sorted on "mo" in ascending order.</p>
  
  <pre><code>df %&gt;%
        select(yr, mo, x:z) %&gt;%
        group_by(yr, mo) %&gt;%
        summarize_each(funs(mean)) %&gt;%
        arrange(yr, mo)
  
  Source: local data frame [5 x 5]
  Groups: yr [3]
  
   yr    mo     x     y     z
  (dbl) (dbl) (dbl) (dbl) (dbl)
  1  2009     1    15     3     2
  2  2009     2    30     6     5
  3  2010     3    45     9     8
  4  2010     4    60    12    11
  5  2011     5    80    16    15
  </code></pre>
  
  <p>How come the "yr" variable won't respond to the desc() fuction but the "mo" variable will?  How do I get the results to be sorted by "yr" in descending order and then "mo" in descending order?  Thanks!</p>