Time graphs: Clarify that times are UTC

[tripleee]

This comes up from time to time, recently here: https://meta.stackoverflow.com/questions/430081/is-there-something-off-with-the-spam-bot#comment995665_430082

The graphs could probably easily have their legends updated to mention that times are in UTC throughout.

(Bonus points for being able to adapt to the user's time zone; though that might be confusing as well?)

[CoconutMacaroon]

I think the easiest solution is probably just adding clarification text that it's UTC. If we're interested in time zones, I (personally) might think something like TIME (UTC) / TIME (XYZ) might be clear. On hover is another option, but I prefer it less as it's somewhat harder to copy-paste.