Bus Plotter IV: Extrapolate From Stop Predictions

I've made the plotter now take advantage of the "predictions" command offered by the webservice that tells you how many minutes it will be before a specific bus arrives at a given stop. Instead of doing linear extrapolation from the previous two reported positions it uses the previous position and an upcoming stop. I first tried using the next stop to do predictions, but because the webservice only provides data at the minute level, this was much too inaccurate and I switched to using a threshold of two minutes out. There might be multiple stops two minutes out, so we break ties by taking the closest one to the last known bus location.

See it in action: mbtaplot