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I don't think Tableau supports crosshairs, but you can get drop lines which will clarify levels for your user.
Highlight the line on your graph
Select Drop lines
Choose Show Drop lines
You'll get something like this:
Hope this helps get you there!
Some problems: this is not an on hover event; the user needs to click on the cell to see the drop lines. I'm not sure if this will be a game-changer for ya.