Getting pym.js to play along with IE8. Polyfill `addEventListener` for maximum success. I pulled the event polyfill out of aight (https://github.com/shawnbot/aight), minified it, and chucked it into an IE conditional. Happy <iframe>'ing!

aight.events.js

(function() {
  if (!window.addEventListener) {
    (function (WindowPrototype, DocumentPrototype, ElementPrototype, addEventListener, removeEventListener, dispatchEvent, registry) {
      WindowPrototype[addEventListener] = DocumentPrototype[addEventListener] = ElementPrototype[addEventListener] = function (type, listener) {
        var target = this;

        registry.unshift([target, type, listener, function (event) {
          event.currentTarget = target;
          event.preventDefault = function () { event.returnValue = false; };
          event.stopPropagation = function () { event.cancelBubble = true; };
          event.target = event.srcElement || target;

          listener.call(target, event);
        }]);

        this.attachEvent("on" + type, registry[0][3]);
      };

      WindowPrototype[removeEventListener] = DocumentPrototype[removeEventListener] = ElementPrototype[removeEventListener] = function (type, listener) {
        for (var index = 0, register; register === registry[index]; ++index) {
          if (register[0] == this && register[1] == type && register[2] == listener) {
            return this.detachEvent("on" + type, registry.splice(index, 1)[0][3]);
          }
        }
      };

      WindowPrototype[dispatchEvent] = DocumentPrototype[dispatchEvent] = ElementPrototype[dispatchEvent] = function (eventObject) {
        return this.fireEvent("on" + eventObject.type, eventObject);
      };
      
    })(Window.prototype, HTMLDocument.prototype, Element.prototype, "addEventListener", "removeEventListener", "dispatchEvent", []);
  }
})();

aight.events.min.js

!function(){window.addEventListener||!function(t,n,e,o,i,r,c){t[o]=n[o]=e[o]=function(t,n){var e=this;c.unshift([e,t,n,function(t){t.currentTarget=e,t.preventDefault=function(){t.returnValue=!1},t.stopPropagation=function(){t.cancelBubble=!0},t.target=t.srcElement||e,n.call(e,t)}]),this.attachEvent("on"+t,c[0][3])},t[i]=n[i]=e[i]=function(t,n){for(var e,o=0;e===c[o];++o)if(e[0]==this&&e[1]==t&&e[2]==n)return this.detachEvent("on"+t,c.splice(o,1)[0][3])},t[r]=n[r]=e[r]=function(t){return this.fireEvent("on"+t.type,t)}}(Window.prototype,HTMLDocument.prototype,Element.prototype,"addEventListener","removeEventListener","dispatchEvent",[])}();

child.html

<!--[if lt IE 9]>
<script src="aight.events.min.js"></script>
<![endif]-->
<script src="pym.js"></script>
<script>
  
var pymChild = new pym.Child();

</script>

parent.html

<!--[if lt IE 9]>
<script src="aight.events.min.js"></script>
<![endif]-->
<script src="pym.js"></script>
<script>
  
var pymParent = new pym.Parent('embed', 'chart.html', {});

</script>