[rrd-users] q:how are decimal places calculated on Y-axis? (how toinfluence)?

[Alex van den Bogaerdt]

----- Original Message ----- 
From: "Pablo Sanchez" <pablo at blueoakdb.com>
To: <rrd-users at lists.oetiker.ch>
Sent: Friday, June 19, 2009 6:04 PM
Subject: [rrd-users] q:how are decimal places calculated on Y-axis? (how 
toinfluence)?


> Howdy,
>
> I have a barometric graph which can display results in hPA or inHG.  I
> store the data in hPA and use a CDEF to convert it to inHG when
> requested.
>
> My problem is that for the graph period, the `inHG' value may vary
> only slightly:
>
>     max:  29.97
>     min:  29.88
>
> I've tinkered with `--y-grid' to get better grid steps (before I was
> only getting one line at 30 and the resulting graphed line was very
> flat.  :):
>
>   --y-grid .01:5
>
> and the line graphs very well.  However, the y-axis labels seem to be
> rounded.  Here's an example of what I see versus what I'd like:
>
>    Wanted     Rendered
>    ------     --------
>    30.05         30
>    30.00         30
>    29.95         30
>    29.90         30
>    29.85         30
>    29.80         30
>
> I've tried using the `--alt-y-grid' option but that didn't help.
>
> I'm using a very old version of RRD on this weather-station laptop:
> 1.2.23
>
> Am I missing an obvious `rrdtool graph' switch or is this a bug in
> this very old version of RRD?  :)


I got intrigued by this question and I just knew I've seen this happen in 
the past. I found version 1.2.23 on my antique fedora machine and wrote this 
small script. Try it.

rrdtool create test.rrd --start 1245448800 DS:test:GAUGE:300:U:U 
RRA:AVERAGE:0:1:10

rrdtool update test.rrd 1245449100:29.97
rrdtool update test.rrd 1245449400:29.96
rrdtool update test.rrd 1245449700:29.95
rrdtool update test.rrd 1245450000:29.94
rrdtool update test.rrd 1245450300:29.95
rrdtool update test.rrd 1245450600:29.96
rrdtool update test.rrd 1245450900:29.97
rrdtool update test.rrd 1245451200:29.98
rrdtool update test.rrd 1245451500:29.99

set `rrdtool graph whatever DEF:test=test.rrd:test:AVERAGE 
PRINT:test:MIN:%6.2lf PRINT:test:MAX:%6.2lf`

rrdtool graph test.png --start 1245448800 --end 1245451500 \
        --title "min $2 max $3" \
        --lower-limit $2 \
        --upper-limit $3 \
        --rigid \
        --alt-y-grid \
        DEF:test=test.rrd:test:AVERAGE \
        LINE1:test#FF0000:test


Both alt-y-grid and rigid seem to be necessary, my initial suggestion of 
using -L was not.

If you don't need the scale to be 'dynamic', you can leave out that 1st 
rrdtool command and just use fixed numbers in the 2nd call.

HTH
Alex