title: make paired, inverted histograms
body: <p>I would like to make paired dotplot histograms for two groups across a set of different tests with the two groups shown in opposite directions on the y-axis. Using this simple data set</p>
  
  <pre><code>dat &lt;- data.frame(score = rnorm(100), group = rep(c("Control", "Experimental"), 50), test = rep(LETTERS[1:2], each=50))
  </code></pre>
  
  <p>I can make faceted dotplots like this</p>
  
  <pre><code>ggplot(dat, aes(score, fill=group)) + facet_wrap(~ test) + geom_dotplot(binwidth = 1, dotsize = 1)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/ETyAe.png" alt="faceted dotplot example"></p>
  
  <p>but I want the Control dots to be pointing down rather than up. Using <a href="https://stackoverflow.com/questions/7579995/facet-grid-of-back-to-back-histogram-failing">this question and answer</a>, I can make a histogram version that looks more or less like what I want</p>
  
  <pre><code>ggplot() + 
    geom_histogram(data=subset(dat, group=="Experimental"), aes(score, fill="Experimental", y= ..count..)) +
    geom_histogram(data=subset(dat, group=="Control"), aes(score, fill="Control", y= -..count..)) +
    scale_fill_hue("Group")
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/AOH9S.png" alt="paired, inverted histograms">
  but now the faceting is gone. I know I could do the faceting manually using <code>grid.arrange</code>, but that would be laborious (my actual data set has many tests, not just 2), is there a more elegant solution?</p>
  
  <p>Two follow-up questions:</p>
  
  <ol>
  <li><code>geom_histogram</code> is giving me a warning that says "Stacking not well defined when ymin != 0". Does anyone know how "not well defined" it is? In other words, is this something I should be concerned about?</li>
  <li>I'd prefer to use dotplot instead of histogram, but the inversion doesn't seem to work for dotplot. Why is that? Any ideas how to get it to work?</li>
  </ol>
  
  <p>Thanks in advance!</p>