Healthy Ride PGH System Flow

Health Ride PGH, Pittsburgh's bike share system, releases quarterly data on their ridership.

The data released includes timestamp that a rider left a particular station, and the time they arrived at a subsequent station. I aggregated these to get a total count of the number of rides that originated from a station and that arrived at a station. In theory, the system works at the bikes just circulate around the system in a balance, but actually, there are "rebalanced" manually throughout the day.

I wanted to see how balanced that flow is in Pittsburgh, or which stations have more departures or arrivals. To estimate this I subtracted the "to" trip from the "from" trips, so a negative number indicates that more rides are originating from that station, and positive numbers mean more trips arrive at that station.
Interestingly, the sum of the "difference" column is not zero, but 147, which may mean bikes were added in the time period, or that some bikes were removed mid-ride for some reason.