private File writeDotSourceToFile(String str) throws java.io.IOException
   {
      File temp;
      try {
         temp = File.createTempFile("graph_", ".dot.tmp", new File(GraphViz.TEMP_DIR));
         FileWriter fout = new FileWriter(temp);
         fout.write(str);
         fout.close();
      }
      catch (Exception e) {
         System.err.println("Error: I/O error while writing the dot source to temp file!");
         return null;
      }
      return temp;
   }