title: Change the layout of ggplot legend
body: <p>I am curious if it is possible to change the layout of the ggplot legend without having to rewrite the build_legend function?</p>
  
  <p>For example: </p>
  
  <p>I want a 2 * 2 layout</p>
  
  <p>a b <br/>
  c d</p>
  
  <p>rather than the normal 1 * 4 or 4 * 1 layout.</p>
  
  <p>a
  b
  c
  d</p>
  
  <p>or </p>
  
  <p>a <br/> b <br/> c <br/> d</p>
  
  <p>Many Thanks,</p>
  
  <p>MK</p>