Add timestamp to diagram on mouse hover

[mariopesch]

Description

I've noticed that only the value is shown in the diagram if a user hovers of the point. Would be great to also display the timestamp when the value was recorded.

Use case

Sometimes you want to get the specific time of a measured value

Implementation ideas (optional)

No response

Additional information (optional)

No response

[jona159]

Done as part of #923