[rrd-users] max / min values not precise? Bug?

[spock]

Hello,

I have difficulties to interpret the graph created with the parameters below
(I omitted some lines, but relevant data should be there).

<http://rrd-mailinglists.937164.n2.nabble.com/file/n7582374/tempmo1.png> 

The graph shows
- one line with high resolution data
- one line with minimum values per day (step = 86400)
- one line with maximum values per day  (step = 86400)

On the graph I do not understand, why the maximum value of the high
resolution data does not correspond with the minimum / maximum values of the
compressed data.

On the first and second day e.g. the max values are higher than any data
point of the actual high resolution data.

Is this a bug? Or is there any magic I do not understand?

I am using 
RRDtool 1.4.7  Copyright 1997-2012 by Tobias Oetiker <tobi at oetiker.ch>
               Compiled Sep  4 2012 23:43:06
on a rasbian raspberry PI.

# rrdtool create temp_pool.rrd --step 900 --start "20140101 00:00" \
# DS:water:GAUGE:1200:-40:50 \
# DS:air:GAUGE:1200:-40:50 \
# DS:roof:GAUGE:1200:-40:80 \
# DS:flow:GAUGE:1200:-40:50 \
# DS:return:GAUGE:1200:-40:50 \
# DS:delta:COMPUTE:return,flow,- \
# DS:pump_flag:GAUGE:1200:0:1 \
# DS:heat_flag:GAUGE:1200:0:1 \
# DS:device:GAUGE:1200:0:90 \
# RRA:AVERAGE:0.5:1:103680 \
# RRA:MIN:0.5:96:3650 \
# RRA:MAX:0.5:96:3650 \
# RRA:AVERAGE:0.5:96:3650



graph_defs='-w 900 -h 525 -D -a PNG -T 15 --slope-mode \
	DEF:water=temp_pool.rrd:water:AVERAGE \
	DEF:air=temp_pool.rrd:air:AVERAGE \
	DEF:delta=temp_pool.rrd:delta:AVERAGE \
	DEF:heat_flag=temp_pool.rrd:heat_flag:AVERAGE \
	DEF:pump_flag=temp_pool.rrd:pump_flag:AVERAGE \
	DEF:roof=temp_pool.rrd:roof:AVERAGE \
	DEF:roof24hmax=temp_pool.rrd:roof:MAX:step=86400 \
	DEF:roof24hmin=temp_pool.rrd:roof:MIN:step=86400 \
	DEF:air24hmax=temp_pool.rrd:air:MAX:step=86400 \
	DEF:air24hmin=temp_pool.rrd:air:MIN:step=86400 \
	CDEF:air24harea=air24hmax,air24hmin,- \
	CDEF:roof24harea=roof24hmax,roof24hmin,- \
'COMMENT:"\t\t      Max      Min                   Max     Min\n" \
        LINE2:roof#0000FF:"Dach norm" \
        LINE2:roof24hmin#FF0000:"Dachmin" \
	AREA:roof24harea#FF000020::STACK \
        LINE2:roof24hmax#00FF00:"Dachmax"'

Thanks for any hint!!



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