title: Extract single plot from ggplot with facet_grid
body: <p>I want to produce some plots using ggplot and <code>facet_grid</code> and save the plot as an object. My problem is that I also want to save each subgroup (i.e. each facet) as an object separately. My question is now if you can extract a single facet from <code>facet_grid</code> and save it as an object? Here is some simple code:</p>
  
  <pre><code>library(ggplot2)
  
  ggplot(data = mtcars, aes(x = disp, y = mpg)) +
    geom_point() +
    facet_grid(. ~ am)
  </code></pre>
  
  <p>Now I'd like to produce two objects - one for <code>am=0</code> and one for <code>am=1</code>.</p>