title: How to control the margins of figures when knitting a HTML in R?
body: <p>I plot a heatmap by heatmap.2 and it looks right in the plots panel of Rstudio. However, when I knit the HTML using knitr, it always choked at: 
  Error in plot.new() : figure margins too large</p>
  
  <p>Anyways to control the figure margin? I reset the out.width and out.height in opts_chunk without success. Thank you.</p>