Finally:
https://github.com/opiesche/Prometheus
The code is anything but pretty, and much of it is still hardcoded (such as the sensor IDs and GPIO pins the sensors and relay are hooked up to). It's working though, so if anyone cares to take a gander, please do so
The web interface is also still rudimentary (and still contains some unmodified elements from the template I used), but it does plot temperatures and pump-on times, shows current pump status and estimates for lag, retainment and time to target temperature, and allows for the target temperature to be set.
I haven't had much time to work on it lately (travel for work got in the way
, but I'll update the repository as soon as I've got the next set of changes ready.