But I could just install the open-source version of Graphite?
You could! Let’s say you take some time to figure out the docs and install it somewhere, then do some tests to get data in and make sure it’s giving you the right data back out. Let’s say it takes a few hours of pretty enjoyable tinkering, because I’m an optimist (your mileage may vary...).
That’s the easy, fun part. Here’s the unfun part: