Graphite supports a timeShift(metric, delta) function which adjusts the time shown by the delta. This is particularly useful for week-over-week/etc comparisons, where you plot one metric against that same metric 7 days ago.
It shouldn't be hard to implement, you just subtract the delta from the timestamp you are looking for before fetching it from the database.
We have this in our internal OpenTSDB as one of the expressions, I'll try to get a PR together for it.
Added timeShift function
Signed-off-by: Chris Larsen <email@example.com>