How to use the vega-dataflow.Graph function in vega-dataflow

var dl = require('datalib'),
    df = require('vega-dataflow'),
    ChangeSet = df.ChangeSet,
    Base = df.Graph.prototype,
    Node  = df.Node, // jshint ignore:line
    GroupBuilder = require('../scene/GroupBuilder'),
    visit = require('../scene/visit'),
    compiler = require('../parse/expr'),
    config = require('./config');

function Model(cfg) {
  this._defs = {};
  this._predicates = {};

  this._scene  = null;  // Root scenegraph node.
  this._groups = null;  // Index of group items.

  this._node = null;
  this._builder = null; // Top-level scenegraph builder.

var dl = require('datalib'),
    df = require('vega-dataflow'),
    ChangeSet = df.ChangeSet,
    Base = df.Graph.prototype,
    Node  = df.Node, // jshint ignore:line
    GroupBuilder = require('../scene/GroupBuilder'),
    visit = require('../scene/visit'),
    config = require('./config');

function Model(cfg) {
  this._defs = {};
  this._predicates = {};

  this._scene  = null;  // Root scenegraph node.
  this._groups = null;  // Index of group items.

  this._node = null;
  this._builder = null; // Top-level scenegraph builder.

  this._reset = {axes: false, legends: false};