add plot stop functional

modules/plot/Plot.js

     return this
   }
 
+  /**
+   *
+   * @return {Plot}
+   */
+  stop() {
+    if (this._currentWorker) {
+      this._currentWorker.stop()
+    }
+    this._currentWorker = null
+    return undefined
+  }
+
   /**
    *
    * @returns {Plot}