graphite-project/graphite-web — pull request #496 by justino (2013-11-14 22:32:10+00:00)
Title: Add graphite URL function: averageLine()

Takes the average of all data points in a series and plots a horizontal
line representing that average.

&format=averageLine(path.to.some.metric)

I was looking for the ability to do this without hacks, and at least one other people that I know of asked about this ability in the IRC chatroom.