title: ggplot2 - stacking not well defined when ymin !=0
body: <p>When plotting the following data set, I get some strange results and was hoping someone could point out the issue.  I am attempting a facet wrap with my data, but it seems that the results are not truly faceted.</p>
  
  <p>Here is my data set <code>type.mean.prod.rev</code></p>
  
  <pre><code>Group.1,            Group.2,           x
  Prod1,         Expired,  0.339658789
  Prod2,         Expired,  0.450215264
  Prod3,         Expired,  0.597100606
  Prod4,         Expired,  0.450625850
  Prod1,   Expiring In 2013,  0.277917631
  Prod2,   Expiring In 2013,  0.122023392
  Prod3,   Expiring In 2013,  0.011988620
  Prod4,   Expiring In 2013,  0.145343114
  Prod1,   Expiring In 2014,  0.386366264
  Prod2,   Expiring In 2014,  0.402404085
  Prod3,   Expiring In 2014,  0.257194254
  Prod4,   Expiring In 2014,  0.396879507
  Prod1, Expiring Post 2015, -0.003942683
  Prod2, Expiring Post 2015,  0.025357259
  Prod3, Expiring Post 2015,  0.133716520
  Prod4, Expiring Post 2015,  0.007151529
  </code></pre>
  
  <p>Here is my plotting attempt</p>
  
  <pre><code>type.mean.prod.rev.bar &lt;- ggplot(type.mean.prod.rev, 
                               aes(x = type.mean.prod.rev$Group.2,
                                   y = type.mean.prod.rev$x)) + 
    geom_bar(bindwidth = 2,
         stat = "identity") + 
    coord_flip() +
    facet_wrap( ~ Group.1, ncol = 4) +
    theme(axis.title.y = element_blank(),
      axis.title.x = element_blank(),
      panel.border = element_rect(color = "black", fill=NA),
      strip.text = element_text(vjust=.7, 
                                 colour="white", 
                                 face="bold", 
                                 size = rel(1.2)),
      strip.background = element_rect(fill = rgb(red=25,
                                                 green = 187,
                                                 blue = 222, 
                                                 maxColorValue = 255),
                                      colour = "black",
                                      size = 1)) + ylab(NULL) + xlab(NULL)
  
  
  type.mean.prod.rev.bar                               
  </code></pre>
  
  <p>The problem with my output is that each level of <code>Group.2</code> is not represented in each panel, but rather each level is in exactly <em>one</em> panel, almost as if the facets are purely aesthetics and the data set is still being treated as whole (<code>Group.1</code> by <code>x</code>).  I know that there is a negative number in <code>x</code>, which may cause trouble...  But even when I reset this value to zero with <code>type.mean.prod.rev[13, 3] &lt;- 0</code> I get the same results.  Anyone know why the output is off?</p>