You can download the google chart API from the following link:
Just hit the URL on your browser and save it by using ctrl+S. And include it in your project.
For more information see here for offline chart API.
this library used two online library also so you need to download then and change the path:
google.loader.ServiceBase = 'https://www.google.com/uds'; google.loader.GoogleApisBase = 'https://ajax.googleapis.com/ajax';
In the above line you can see those libraries.