How to use the vega-scenegraph.render.canvas function in vega-scenegraph

prototype.toImageURL = function(type) {
  var v = this, Renderer;

  // lookup appropriate renderer
  switch (type || 'png') {
    case 'canvas':
    case 'png':
      Renderer = sg.canvas.Renderer; break;
    case 'svg':
      Renderer = sg.svg.string.Renderer; break;
    default: throw Error('Unrecognized renderer type: ' + type);
  }

  var retina = sg.canvas.Renderer.RETINA;
  sg.canvas.Renderer.RETINA = false; // ignore retina screen

  // render the scenegraph
  var ren = new Renderer(v._model.config.load)
    .initialize(null, v._width, v._height, v._padding)
    .background(v._bgcolor)
    .render(v._model.scene());

  sg.canvas.Renderer.RETINA = retina; // restore retina settings

  // return data url
  if (type === 'svg') {
    var blob = new Blob([ren.svg()], {type: 'image/svg+xml'});
    return window.URL.createObjectURL(blob);
  } else {
    return ren.canvas().toDataURL('image/png');

prototype.toImageURL = function(type) {
  var v = this, Renderer;

  // lookup appropriate renderer
  switch (type || 'png') {
    case 'canvas':
    case 'png':
      Renderer = sg.canvas.Renderer; break;
    case 'svg':
      Renderer = sg.svg.string.Renderer; break;
    default: throw Error('Unrecognized renderer type: ' + type);
  }

  var retina = sg.canvas.Renderer.RETINA;
  sg.canvas.Renderer.RETINA = false; // ignore retina screen

  // render the scenegraph
  var ren = new Renderer(v._model.config.load)
    .initialize(null, v._width, v._height, v._padding)
    .background(v._bgcolor)
    .render(v._model.scene());

  sg.canvas.Renderer.RETINA = retina; // restore retina settings

  // return data url
  if (type === 'svg') {
    var blob = new Blob([ren.svg()], {type: 'image/svg+xml'});
    return window.URL.createObjectURL(blob);
  } else {
    return ren.canvas().toDataURL('image/png');
  }

var sg = require('vega-scenegraph').render,
    canvas = sg.canvas,
    svg = sg.svg.string,
    View = require('./View');

function HeadlessView(width, height, model) {
  View.call(this, width, height, model);
  this._type = 'canvas';
  this._renderers = {canvas: canvas, svg: svg};
}

var prototype = (HeadlessView.prototype = new View());

prototype.renderer = function(type) {
  if(type) this._type = type;
  return View.prototype.renderer.apply(this, arguments);
};