title: ggsave() options for grid arrangements in ggplot
body: <p><code>ggsave()</code> doesn't seem to work with the grid package (see below). How do I save this combination of plot <code>p1</code> and plot <code>p2</code>. The following code only save the last plot <code>p2</code> that ggplot() sees.</p>
  
  <pre><code>library(tidyverse)
  p1 &lt;- ggplot(mpg, aes(fl)) + geom_bar()
  p2 &lt;- ggplot(mpg, aes(cty, hwy)) + geom_col()
  grid.newpage()
  grid.draw(rbind(ggplotGrob(p1), ggplotGrob(p2), size = "last"))
  ggsave("mpg.png")
  </code></pre>