[Idea] Add time graphs annotations

[khasinski]

Use chart-js annotations to overlay data on multiple time graphs at the same time.

This allows to quickly correlate events (both points in time and time ranges) with multiple graphs. Especially useful in dashboards. I've used this feature extensively in a clients code to see the influence of deployments and outages on business metrics.

Here is a demo screenshot:

And here is a PR: #402

[khasinski]

@ankane any opinion on this? :)