title: geom_area not stacking (ggplot)
body: <p>Here is the structure of my data:</p>
  
  <pre><code>Classes ‘tbl_df’, ‘tbl’ and 'data.frame':   60 obs. of  3 variables:
  $ year     : num  1990 1990 1991 1992 1993 ...
  $ studyType: Factor w/ 4 levels "meta","observational",..: 2 3 3 1 3 3 3 1 3 3 ...
  $ N        : int  1 4 4 1 2 5 3 1 2 6 ...
  </code></pre>
  
  <p>I'm trying to get this into a stacked area chart using the following code:</p>
  
  <pre><code>ggplot(evidence.summary.main, aes(x = year, y = N, fill=studyType)) + 
  geom_area(alpha=.80) 
  </code></pre>
  
  <p>Any ideas why I cannot get this to stack properly?  </p>
  
  <p><img src="https://i.stack.imgur.com/nExvm.jpg" alt="enter image description here"></p>
  
  <p>And my data from <code>dput</code>: </p>
  
  <pre><code>structure(list(year = c(1990, 1990, 1991, 1992, 1993, 1994, 1995, 
  1996, 1996, 1997, 1997, 1998, 1998, 1999, 1999, 2000, 2000, 2000, 
  2001, 2001, 2002, 2002, 2003, 2003, 2003, 2004, 2004, 2004, 2005, 
  2005, 2006, 2006, 2006, 2007, 2007, 2007, 2007, 2008, 2008, 2008, 
  2009, 2009, 2009, 2009, 2010, 2010, 2010, 2010, 2011, 2011, 2011, 
  2011, 2012, 2012, 2012, 2012, 2013, 2013, 2013, 2013), studyType = structure(c(2L, 
  3L, 3L, 1L, 3L, 3L, 3L, 1L, 3L, 3L, 4L, 1L, 3L, 1L, 3L, 1L, 2L, 
  3L, 1L, 3L, 1L, 3L, 1L, 3L, 4L, 1L, 2L, 3L, 3L, 4L, 1L, 3L, 4L, 
  1L, 2L, 3L, 4L, 1L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 
  2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L), .Label = c("meta", 
  "observational", "rct", "sr"), class = "factor"), N = c(1L, 4L, 
  4L, 1L, 2L, 5L, 3L, 1L, 2L, 6L, 1L, 2L, 2L, 1L, 5L, 3L, 1L, 1L, 
  4L, 6L, 1L, 10L, 2L, 6L, 3L, 1L, 2L, 8L, 12L, 2L, 3L, 12L, 4L, 
  3L, 1L, 9L, 5L, 4L, 7L, 4L, 4L, 1L, 16L, 8L, 9L, 2L, 17L, 7L, 
  6L, 2L, 15L, 16L, 3L, 2L, 19L, 14L, 8L, 2L, 28L, 20L)), row.names = c(NA, 
  -60L), .Names = c("year", "studyType", "N"), class = c("tbl_df", 
  "tbl", "data.frame"))
  </code></pre>