Is this still a problem? If so could you post some screenshots of your mapping and JSON structure?
Did you find the documentation https://docs.mendix.com/refguide/charts-any-configuration#7-events
Maybe we can add a section in the how-to about the tooltip https://docs.mendix.com/howto/extensibility/charts-any-usage
You can also have a look at a sample the development TestProject.mpk for charts, which could be found on the GitHub repo