title: Knitr error: Error in .External2(C_X11, paste("png::", filename, sep = ""), g$width, : unable to start device PN
body: <p>Something similar to this question has been asked before but the answer might not apply in my case.  </p>
  
  <p>I am getting the following error after trying to "knit" a document:</p>
  
  <pre><code>Quitting from lines 30-34 (test.Rnw)
  Error in .External2(C_X11, paste("png::", filename, sep = ""), g$width,  :
    unable to start device PNG
  In addition: Warning message:
  In (function (filename = "Rplot%03d.png", width = 480, height = 480,  :
    unable to open connection to X11 display '
  </code></pre>
  
  <p>It just happens with the following: </p>
  
  <p>&lt;>=
      library(lattice)
      bitmap('test.png')
      xyplot(1:2~1:2)
      dev.off()
  @</p>
  
  <p>The system that I am running this in does not have X11 installed.  Is that absolutely necessary??  This seems to be what is required (according to my online research) but I was hoping not to have to do it as this is a production machine and I wanted to keep it as "lean" as possible. 
  Thank you very much for any help. 
  Ramiro</p>