title: Set number of columns (or rows) in a facetted plot
body: <p>I have a facetted plot like this:</p>
  
  <pre><code>ggplot(mtcars, aes(x = hp, y = mpg)) +
    geom_point() +
    facet_grid(. ~ carb)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/mtmlU.png" rel="noreferrer"><img src="https://i.stack.imgur.com/mtmlU.png" alt="enter image description here"></a>
  However, the graph is too wide to be clearly read. </p>
  
  <p>I'd like to be able to take the three rightmost locations and place them under the three leftmost, i.e. the facets should be in three columns * two rows like this.</p>
  
  <blockquote>
  <pre><code>1   2   3
  
  4   5   6
  </code></pre>
  </blockquote>
  
  <p>Is it possible to set the layout of the facets, i.e. to set number of columns (or rows) with <code>facet_grid()</code>?</p>
  
  <p>The documentation on <a href="http://ggplot2.tidyverse.org/reference/facet_grid.html" rel="noreferrer">facet_grid</a> doesn't seem to indicate that it's possible.</p>
  
  <p>Thanks for the help :-)</p>