What scale or measure does Mavericks' and Yosemite's "memory pressure" adhere to?

Mavericks' (and also Yosemite's) Activity Monitor shows a new diagram, the _memory pressure_. Sadly, its help text only vaguely explains what exactly it measures. How is memory pressure calculated? _Picture creds go to [this answer](https://apple.stackexchange.com/a/106071/12643) from a poll question on Mavericks' best new feature._