title: Produce two plots from same chunk / statement in knitr
body: <p>Is it possible to have plot-generating code output two versions of the same figure, at different sizes, from a <code>.Rmd</code> document? Either through chunk options (I didn't see anything that works directly here), or through a custom <code>knitr</code> hook? Preferably this would be done with the <code>png</code> device.</p>
  
  <p>My motivation: I'd like to be able to output a figure at one size, which would fit inline in a compiled HTML document, and another figure that a user could show after clicking (think <a href="http://www.fancybox.net/">fancybox</a>). I think I'll be able to handle the scripting necessary to make that work; however, first I need to convince R / <code>knitr</code> to output two versions of the figure.</p>
  
  <p>Although I'm sure there are workarounds, it would be best if there was some way to get it to 'just work' behind the scenes, e.g. through a <code>knitr</code> hook. That way, we don't have to do anything special to the R code within a chunk, we just modify how we parse / evaluate that chunk.</p>
  
  <p>Alternatively, one could use SVG graphics that would scale nicely, but then we lose the nice inference of good sizes for the plot labels, and vector graphics aren't great for plots with many many points.</p>