[BioC] qrqc with error message of "unable to start device PNG" and "unable to open connection to X11 display"?

[Sang Chul Choi]

Hi,

I am trying to use qrqc Bioc package in a linux machine used as a compute node.  I have the following error:

====================
Error in X11(paste("png::", filename, sep = ""), g$width, g$height, pointsize,  : 
  unable to start device PNG
In addition: Warning message:
In grDevices::png(..., width = width, height = height, res = dpi,  :
  unable to open connection to X11 display ''
====================

I have googled for an answer and bumped into following mailing list posts:

https://stat.ethz.ch/pipermail/r-help/2008-March/155943.html
https://stat.ethz.ch/pipermail/r-help/2008-February/155021.html
https://stat.ethz.ch/pipermail/r-help/2008-February/155023.html

It is not obvious what I should do. I will appreciate your answers.

Thank you,

SangChul