Dygraphs sync fix test - FIX

HTML

<link rel="stylesheet" href="https://unpkg.com/dygraphs@2.2.1/dist/dygraph.min.css">
<script src="https://unpkg.com/dygraphs@2.2.1/dist/dygraph.js"></script>
<div id="graph1" class="graph"></div>
<div id="graph2" class="graph"></div>
<div id="graph3" class="graph"></div>

CSS

.graph {
  width: 80%;
  height: 200px;
}

JavaScript

var Dygraph;
  if (window.Dygraph) {
    Dygraph = window.Dygraph;
  } else if (typeof module !== 'undefined') {
    Dygraph = require('../dygraph');
    if (typeof Dygraph.NAME === 'undefined' && typeof Dygraph["default"] !== 'undefined') Dygraph = Dygraph["default"];
  }
  /* end of loader wrapper header */

  var synchronize = function synchronize(/* dygraphs..., opts */
  ) {
    if (arguments.length === 0) {
      throw 'Invalid invocation of Dygraph.synchronize(). Need >= 1 argument.';
    }
    var OPTIONS = ['selection', 'zoom', 'range'];
    var opts = {
      selection: true,
      zoom: true,
      range: true
    };
    var dygraphs = [];
    var prevCallbacks = [];
    var parseOpts = function parseOpts(obj) {
      if (!(obj instanceof Object)) {
        throw 'Last argument must be either Dygraph or Object.';
      } else {
        for (var i = 0; i < OPTIONS.length; i++) {
          var optName = OPTIONS[i];
          if (obj.hasOwnProperty(optName)) opts[optName] = obj[optName];
        }
      }
    };
    if (arguments[0] instanceof Dygraph) {
      // Arguments are Dygraph objects.
      for (var i = 0; i < arguments.length; i++) {
        if (arguments[i] instanceof Dygraph) {
          dygraphs.push(arguments[i]);
        } else {
          break;
        }
      }
      if (i < arguments.length - 1) {
        throw 'Invalid invocation of Dygraph.synchronize(). ' + 'All but the last argument must be Dygraph objects.';
      } else if (i == arguments.length - 1) {
        parseOpts(arguments[arguments.length - 1]);
      }
    } else if (arguments[0].length) {
      // Invoked w/ list of dygraphs, options
      for (var i = 0; i < arguments[0].length; i++) {
        dygraphs.push(arguments[0][i]);
      }
      if (arguments.length == 2) {
        parseOpts(arguments[1]);
      } else if (arguments.length > 2) {
        throw 'Invalid invocation of Dygraph.synchronize(). ' + 'Expected two arguments: array and optional options argument.';
    ...