DataPoint generic type now types data proptery as T instead of T or undefined

gist-charts/source/classes/datapoint.class.ts

@@ -7,7 +7,7 @@ export class DataPoint<T = any> {
      * @type {(T | undefined)}
      * @memberof DataPoint
      */
-    public data: T | undefined;
+    public data: T;
 
     /**
      * Gist Chart use only.