[rrd-users] Combining Multi-RRD Datapoints in Graph

Simon Hobson linux at thehobsons.co.uk
Fri Feb 13 15:14:30 CET 2009

William R. Lorenz wrote:

>I'm looking to take 2-4 unique RRD files that plot the same data from
>different perspectives (i.e. temperature readings from four different
>sensors) and average all the data points together into one graph line.
>Is there a CDEF, VDEF, or other such function that will allow me to
>average more than one data source together for this rrdgraph purpose?

You can pull data from more than one source - just define multiple 
data sources with the right files.

Averaging four values to one line is basic arithmetic, if the values 
are a to d, then the average is simply (a+b+c+d)/4, or in rpn 
"a,b,+,c,+,d,+,4,/", or "a,b,c,d,+,+,+,4,/".

If one or more of the sources may be undefined (or null), then the 
result will be null. Working around this is a bit more involved.

Simon Hobson

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