title: Force facet_wrap to fill bottom row (and leave any "gaps" in the top row)
body: <p>I'd like to force facet_wrap to fill from the top-left, but in such a way as to always fully fill the bottom row.  (That is, from the top-left plus whatever horizontal offset is required to fully fill the bottom row.)</p>
  
  <pre><code>library(ggplot2)
  
  n &lt;- 1000
  df &lt;- data.frame(x=runif(n), y=rnorm(n), label=sample(letters[1:7], size=n, replace=TRUE))
  df$label.rev &lt;- factor(df$label, levels=sort(unique(df$label), decreasing=TRUE))
  
  p &lt;- ggplot(df, aes(x=x, y=y)) + geom_point()
  p1 &lt;- p + facet_wrap(~ label, ncol=3)
  p2 &lt;- p + facet_wrap(~ label, ncol=3, as.table=FALSE)
  p3 &lt;- p + facet_wrap(~ label.rev, ncol=3, as.table=FALSE)
  </code></pre>
  
  <p>p1:  I'm happy with the left-to-right, top-to-bottom ordering of the labels, but I'd like the gap to be at the top left instead of the bottom right.</p>
  
  <p>p2:  Gap is now in the top row (top right rather than top left, unfortunately), but the label order is wrong.</p>
  
  <p>p3:  Similar to p2; attempts to fix the label order, and fails.</p>
  
  <p>I'd like the facets to be ordered like this:</p>
  
  <pre><code>_ _ A
  B C D
  E F G
  </code></pre>
  
  <p>...so that there are axes along the entire bottom row, and because I think it would look better than the default.  How can I do this?</p>