The history graph card, when hovering used to show the start time, end time, and duration, for state changes (eg on/off). I'm not sure when it began but it now only shows the start time. This is much less useful.
#History graph state end & duration missing
swift violet
tame cave
I think it's actually cut off from what I can tell. The start time is visible, and in the DOM the endtime and duration are there but below the bounds of the card.
That'd be a good issue to raise in https://github.com/home-assistant/frontend