I usually just manually type "1.00" or "1.50" as tip for a short city fare (under $15), And "5.00" for a long trip (airport). I guess I'm cheap.

I would also be interested in a histogram of raw tip amounts to see if people cluster at 1.00, 1.50, or 2.00 as well. We could even bucket by fair sizes.

It would also be very cool if we had metadata on the credit card, for example, do corporate cards tip a lot better than consumer cards. Presumably company-paid travel leads to better tips.