title: How to expand axis asymmetrically with ggplot2 without setting limits manually?
body: <p>I want the bottom axis for a barplot to be zero, but top expanded by a small amount (according to some scale). However, I am plotting the output of <code>stat_summary</code> so I do not have the maximum value available to me, and I am using facets with <code>scale="free_y"</code> so I cannot manually set them. Otherwise I would use <a href="https://stackoverflow.com/questions/20220424/ggplot2-bar-plot-no-space-between-bottom-of-geom-and-x-axis-keep-space-above">baptiste's trick of adding <code>geom_blank</code></a>. I know I could read off the values and create a data frame with upper bounds manually, but can this be set without having to hard-code the limits? </p>
  
  <p>The other option is to compute the means outside of the function call and just plot that without calling <code>stat_summary</code> (this way I have the upper bounds directly for each facet), but is there any way around this?</p>
  
  <p>Example:</p>
  
  <pre><code>ggplot(mtcars)+
    stat_summary(aes(cyl,mpg),
                 fun.y="mean",
                 geom="bar")+
    scale_y_continuous(expand=c(0,0))+
    facet_grid(carb~.,"free_y")
  </code></pre>