I think having an annotation feature that lets you put some extra data available on the chart would be very helpful. It could be a target with callbacks similar to tooltips, so you could choose to have extra data displayed on hover. There are a lot of different ways to make annotations, here are a few:
Horizontal / Vertical Lines
Google Charts has implemented this by adding
Role as an option –https://developers.google.com/chart/interactive/docs/roles?hl=en. @etimberg & @tannerlinsley – I think we should seriously take a look at how we plan on supporting those options beyond just annotations while we still have some API flexibility.