title: Alter just horizontal spacing between facets (ggplot2)
body: <p><code>ggplot2</code> has the ability to change the margins between a faceted plot using the argument <code>panel.margin</code> in <code>opts</code>.  This seems to change both horizontal and vertical spacing.  Is there a way to change the spacing of either horizontal or vertical without changing the other?</p>
  
  <p><strong>An example with outcome and desired outcome:</strong></p>
  
  <pre><code>mtcars[, c("cyl", "am", "gear")] &lt;- lapply(mtcars[, c("cyl", "am", "gear")], as.factor)
  
  p &lt;- ggplot(mtcars, aes(mpg, wt, group = cyl)) + 
      geom_line(aes(color=cyl)) +
      geom_point(aes(shape=cyl)) + 
      facet_grid(gear ~ am) +
      theme_bw()        
  
  p + opts(panel.margin = unit(1, "lines")) 
  </code></pre>
  
  <p>So it currently looks like:<img src="https://i.stack.imgur.com/Da25Y.png" alt="enter image description here"></p>
  
  <p>How can we make it look more like:<img src="https://i.stack.imgur.com/9vmie.png" alt="enter image description here"></p>