title: How to resize and save plots in png format?
body: <p>I would like to plot the results from a quantile regression, but am not able to:</p>
  
  <ol>
  <li>control the dimensions/size of the plots and</li>
  <li>save the plots as png.</li>
  </ol>
  
  <p>Here is my code:</p>
  
  <pre><code>require(quantreg)
  data(engel)
  attach(engel)
  xx &lt;- income - mean(income)
  zz &lt;- c(120, diff(income))
  fit1 &lt;- summary(rq(foodexp~xx+zz, tau=2:98/100))
  </code></pre>
  
  <p>Then:</p>
  
  <pre><code>png('res.png')
  plot(fit1, mfrow=c(1,2))
  </code></pre>
  
  <p>Only the zz plot is saved to the res.png file.. Is there any way I can save the plots in separate files (two and one)? 
  and how do I control the width/height of the plots? I like all the individual plots to have width=height (square) when i save them to the .png file? </p>