message: upadte README.md and fix to have a default  value properly.
diff: @@ -52,17 +52,19 @@ function load(url, arg, prop, el, callback) {
  // Embed a Vega visualization component in a web page.
  // el: DOM element in which to place component (DOM node or CSS selector)
  -// opt: Embedding specification (parsed JSON or URL string)
  +// spec : String : A URL string from which to load the Vega specification.
  +// Object : The Vega/Vega-Lite specification as a parsed JSON object.
  +// opt: A JavaScript object containing options for embedding.
  // callback: invoked with the generated Vega View instance
  function embed(el, spec, opt, callback) {
  var cb = callback || function(){}, source,
  - renderer = opt.renderer || 'canvas',
  - actions = opt.actions || {}, mode;
  + renderer = (opt && opt.renderer) || 'canvas',
  + actions = (opt && opt.actions) || {}, mode;
  opt = opt || {};
  try {
  // Load the visualization specification.
  if (vega.isString(spec)) {
  - return load(spec, opt, 'source', el, callback);
  + return load(spec, opt, 'url', el, callback);
  } else {
  source = JSON.stringify(spec, null, 2);
  }
  