[R] How to build a package which loads Rgraphviz (if installed)...

[Søren Højsgaard]

Dear List,
I use Rgraphviz for display of graphs in some packages. Since Rgraphviz is no longer on CRAN it needs to be installed from Bioconductor and that is fine, but I have trouble figureing out the following: I create a plot method which - if Rgraphviz is installed - uses Rgraphviz for displaying and otherwise does nothing. This is implemented as:
 
 
  if (!("package:Rgraphviz" %in% search())){
    if("Rgraphviz" %in% installed.packages()){
      require(Rgraphviz)
    } else {
      cat("The Rgraphviz package (from Bioconductor) must be installed to display the models\n")
      return()
    }
  }
 ... else go on and do the plotting...
 
When I run rcmd check packagename (on Windows XP, using R.2.7.1) I get 
 
* checking for unstated dependencies in R code ... WARNING
'library' or 'require' calls not declared from:
  Rgraphviz
See the information on DESCRIPTION files in the chapter 'Creating R
packages' of the 'Writing R Extensions' manual.

I can not state Rgraphviz in the Depends-field of the DESCRIPTION file because then the package does not pass the windows checks for going on CRAN. I can not see any solution from "writing R extensions", but I apologize if I have overlooked it. Does anyone have a suggestion on what to do?
 
Cheers
Søren