title: knitr not aligning figures to center in pdf output?
body: <p>Using knitr to make pdfs, the figures don't show when using the fig.align='center' option:</p>
  
  <pre><code>require(knitr)
  opts_chunk$set(fig.align='center') 
  </code></pre>
  
  <p>OR</p>
  
  <pre><code>```{r chunkname, fig.align='center'}
  ...code that makes figure...
  ```
  </code></pre>
  
  <p>Either way, no figures come out on the pdf when pressing the knit PDF button. But I remove the fig.align option and the figures appear, left aligned.</p>
  
  <p>This was an old problem that should have been solved with the 1.8 knitr version, but still have the same problem :(</p>
  
  <p>Any ideas?</p>
  
  <p>Thank you!</p>