The function generates a dot-text file as supported by Graphviz library. 
In order to draw a graph the user needs a dot-file converter and pdf/postscript viewer. 
The environment variables R_DOTVIEWER and R_PDFVIEWER specify the corresponding executable routines.
If Graphviz is installed and the variable R_DOTVIEWER is set with the full 
path to the dot executable file (the routine that converts a dot-text file to a postscript or pdf), 
a pdf or postscript file is created depending on the value of the format parameter.
If the file variable is not specified, then the function just prints out the resulting string  
which otherwise would be written into a dot file. 
Next, if a pdf-viewer is available, the created postscript or pdf file is shown.