Throw an error from abstract method `Core.prototype.getDataRange` (see #966)

lib/timeline/Core.js

@@ -480,11 +480,7 @@ Core.prototype.fit = function(options) {
  */
 Core.prototype.getDataRange = function() {
   // must be implemented by Timeline and Graph2d
-
-  return {
-    min: null,
-    max: null
-  }
+  throw new Error('Cannot invoke abstract method getDataRange');
 };
 
 /**