I’ve done many presentations about the WPGraphQL plugin, and I find myself showing demos of many of the same features, so in order to streamline the demos before my presentation at the 2018 WordCamp Phoenix, I created https://playground.wpgraphql.com
This site showcases examples of GraphQL Queries and Mutations on a live WordPress site with the WPGraphQL plugin active. The demos start out very basic, and as you work down the list, you will see progressively more advanced examples of GraphQL use cases, demonstrating both unique features of the WPGraphQL plugin and the broader GraphQL Query Language.
Enjoy.