[rrd-users] Re: Graph data is affected by the size of PNG image

Simon Hobson linux at thehobsons.co.uk
Thu Jan 4 17:51:58 MET 2007

Alejandro Galue wrote:

>Graph 1
>Graph 2
>The real data is showed on "Graph 2" (the big one); but "Graph 1" (the
>small one) shows non valid data.
>If you see Graph 1, almost all peaks dissapears from graph.
>I used "reduce=MAX" on DEF lines but this does not work.

Graph 1 looks very much like the data from graph 2 reduced to a lower 
resolution by an averaging function. Chances are that the start & end 
times for your graphs are causing the RRD tool to select a different 
set of data from your archive than you were expecting.

It is a good idea to select your end time as a multiple of the step 
size for the resolution of the data you want to plot, as in end = end 
- ( end % step ). Then select your start time as the end time minus 
the step size times the number of pixels (or should that be "number 
of pixels - 1" ?).

If you want to show max values on a graph then you must have a max 
value aggregation in your rrd. It is impossible to infer the max 
values from an average !

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