code: def render_output_json(self, graph_data, output_file=None):
          
          if output_file:
              with open(output_file, 'wt') as json_output_f:
                  json.dump(graph_data, json_output_f)
          else:
              self.stdout.write(json.dumps(graph_data))
docs: Write model data to file or stdout in JSON format.