title: Centre a plot to the middle of a page using Knitr
body: <p>I'd like to align a plot to the center of a page of a knitr-generated pdf document. I can horizontally align the plot to the center using <code>fig.align='center'</code> but can't figure out how to get the plot vertically aligned to the center.</p>
  
  <p>I've been using the following code:</p>
  
  <pre><code>---
  header-includes: \usepackage{graphicx}
  output: 
    pdf_document
  geometry:
    left=1in,right=1in,top=1in,bottom=1in
  ---
  
  
  ```{r,fig.align='center',out.extra='angle=90', echo=FALSE}
  
  library(ggplot2)
  ggplot(diamonds, aes(y=carat, x=price, colour=clarity))+geom_point()+
  facet_wrap(~cut)
  
  ```
  </code></pre>