1. People aren't tipping
2. Cab drivers aren't reporting their tips.
Being a native new yorker, legality aside, I'd bet most cash tips go unreported. Reporting a tip makes that tip taxable, so there is a very strong incentive to bury it if they think they can avoid trouble or suspicion.
Then you can filter the data to show only CC transactions. There might be some additional variance of CC vs cash tipping, but I think the overall trend will still be there in just the CC transactions.
1. People aren't tipping 2. Cab drivers aren't reporting their tips.
Being a native new yorker, legality aside, I'd bet most cash tips go unreported. Reporting a tip makes that tip taxable, so there is a very strong incentive to bury it if they think they can avoid trouble or suspicion.