Hey Charlie! We've been back and forth on this in the lifetime of the startup, but we're now doing all metrics ourselves, so it is internally built. I built an early version of the cohort analysis (which didn't scale, but that didn't matter) just a few months in and it was massively valuable. It's not too hard to knock something together.
Thanks for the answer Joel!
By the way - I'm curious why you guys had to built this internally - I always get the feeling people prefer using a 3rd party service like kissmetrics than building something.