[rrd-users] VRULE for sunset, sunrise

[Jason Fesler]

> I want it in the RRD, so I can plot it as a graph, for the
> year graph, but I'd like to show the day/night somehow for
> any particular day.

At whatever interval you store regular data, do a lookup and also store 
into RRD, whether it is day or night (perhaps as 1 and 0, or 1 and NaN?).
Using that, you can have have an RRD graph that loads your data, + day or 
night, and a CDEF can make a choice what to show based on that.

That'll make it so that you only have to worry about the changing time for 
sunrise/sunset at the time of insert, and not for any of your later 
graphs.