For a recent project retrospective we wanted to chart some metrics over the course of the entire project. Things like number of unit tests, test coverage, how long builds took to run, number of failing tests, etc. Taken in isolation these metrics aren't incredibly exciting, but when you plot them over time and hold that against other metrics like team morale, story point velocity, open defects, etc then often some interesting correlations can emerge.
So, we wanted metrics for internal quality, but we actually hadn't done the best job at collecting those metrics, particularly at the start of the project. So under pressure to get ready for our project retro we decided that we'd have to pass on graphing the internal quality metrics.
After the retrospective - which was a very valuable exercise regardless - I decided to figure out a way to capture this kind of data retroactively. I reasoned that if we had a script which generated a metric for the codebase at an instance in time then we can easily leverage The Awesome Power Of Git™ to collect that same metric over an entire set of commits.