title: Simpler population pyramid in ggplot2
body: <p>I want to create a population pyramid with ggplot2. This question was asked <a href="https://stackoverflow.com/questions/14245361/population-pyramid-density-plot-in-r">before</a>, but I believe the solution must be far simpler.</p>
  
  <pre><code>test &lt;- (data.frame(v=rnorm(1000), g=c('M','F')))
  require(ggplot2)
  ggplot(data=test, aes(x=v)) + 
      geom_histogram() + 
      coord_flip() + 
      facet_grid(. ~ g)
  </code></pre>
  
  <p>Produces this image. In my opinion, the only step missing here to create a population pyramid is to invert the x axis of the first facet, so that is goes from 50 to 0, while keeping the second untouched. Can anyone help?</p>
  
  <p><img src="https://i.stack.imgur.com/WtbnT.png" alt="Population pyramid"></p>