[rrd-users] Graphing inconsistencies graphing different time slices

Simon Hobson linux at thehobsons.co.uk
Mon Jun 8 15:47:49 CEST 2015

Munroe Sollog <mus3 at lehigh.edu> wrote:

> I have an inconsistency when I graph the same event over 10days versus 48hrs.  I would expect
> because I am using MAX that the peak on the 48hr graph and the 10day graph for the *same* event
> would still read the same number.  However, the peak for Sunday noon on the 48hr graph is at 600,
> and the peak for that same period on the 10day graph is *higher* at 1.1k.  I believe I have
> included all the necessary files below.  I didn't want to paste the raw info here as it would be
> very large.  Any insight would be appreciated.

You are adding up different max values. Consider this set of data :
a1 = 1 5 3 7 4
b1 = 2 6 9 1 4

If you do a graph of max (max a + max b) without any consolidation then you get the values :
m1 = 2 11 12 8 8

If you consolidate 5 values into one, you then get :
a2 ave = 4
a2 max = 7
b2 ave = 4.4
b2 max = 9

m2 (= max (max a2 + max b2)) = 16

So when you draw a graph over a longer time period, depending on the consolidation, you may see your maximum value go up (with these made up figures, by 1/3) when you add max values together.

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