fixed error caused by used with ExtJS

src/core.js

@@ -10,6 +10,7 @@ function Chart(config) {
     // bind "this" to nested API
     (function bindThis(fn, target, argThis) {
         for (var key in fn) {
+            if (key === '$extIsFunction') { continue; }
             target[key] = fn[key].bind(argThis);
             if (Object.keys(fn[key]).length > 0) {
                 bindThis(fn[key], target[key], argThis);