title: How to change knitr options mid chunk
body: <p>Hi I would like to change chunk options, mid chunk, without having to create a new chunk..</p>
  
  <p>running the following code I would expect to get two very different size outputs, but for some reason this does not seem to be the case.</p>
  
  <p>Also the second plot doesn't plot at all...(it does when you change it to plot(2:1000)...but either way the second output is the same size as the first. both <code>fig.width=7</code>. What am I doing wrong?</p>
  
  <p>Pls note the importance of 'mid chunk' the reason for this is that I would like to change the chunk options several times when running a function to get different outputs of different sizes.</p>
  
  <pre><code>```{r}
  sessionInfo()
  
  opts_chunk$set(fig.width=3)
  
  plot(1:1000)
  
  opts_chunk$set(fig.width=10)
  
  plot(1:1000)
  
  ```
  </code></pre>
  
  <p>the sessionInfo output is as follows:</p>
  
  <pre><code>## R version 2.15.1 (2012-06-22)
  ## Platform: i386-pc-mingw32/i386 (32-bit)
  ## 
  ## locale:
  ## [1] LC_COLLATE=English_United Kingdom.1252 
  ## [2] LC_CTYPE=English_United Kingdom.1252   
  ## [3] LC_MONETARY=English_United Kingdom.1252
  ## [4] LC_NUMERIC=C                           
  ## [5] LC_TIME=English_United Kingdom.1252    
  ## 
  ## attached base packages:
  ## [1] stats     graphics  grDevices datasets  utils     methods   base     
  ## 
  ## other attached packages:
  ## [1] knitr_0.7
  ## 
  ## loaded via a namespace (and not attached):
  ## [1] digest_0.5.2   evaluate_0.4.2 formatR_0.5    parser_0.0-16 
  ## [5] plyr_1.7.1     Rcpp_0.9.13    stringr_0.6    tools_2.15.1  
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/7tpjL.jpg" alt="html preview"></p>