google

Google charts is one of the services that has fantastic documentation, allowing you to really dig into it and get the best possible result. With a wide variety of chart types available and configurable options Google welcomes you to dive into it's API and see what you can do.

Google has a dazzling array of free and premium services (yes they do more them email, maps, and analytics) and the Drupal community, being the open source wonder that it is, has built upon this and created multiple modules you can use to bring those services into Drupal.