dynamicClassName.js

var className = 'Graph' + type;

// lesser of two uglies?

var graph = new window[className](this);
// versus
var graph = eval('new ' + className + '(this)');

// using window makes sanity checking easier:
if (typeof window[className] === 'function') // instantiate ...
// versus
if (eval('typeof ' + className) === 'function') // instantiate ...