[rrd-users] Re: Two independent Y-axis scales

Robert Nelson rnelson at windchannel.com
Fri Oct 29 13:38:14 MEST 2004

>> I am using RRDTool with Nagios and am tracking some info on radios. One
>> the values is RSSI, which can range from 0 to 2500. The other is jitter,
>> which has a range of 0 to 15. I'd like to make two Y-axis scales, Blue
>> from 0 to 2500 and Green from 0 to 15, with the appropriate colored
>> lines for the values. As it is, the jitter just shows as a solid green
>> line on the bottom of the chart, so it's really hard to tell if it's
>> varying at all.
>> I can do this as multiple charts, but it would be really nice to view
>> all this stuff in one place. We've used other tools that do this in the
>> past and it works great, since both lines really should be a solid
>> horizontal line (should be...). I looked through the archive and saw a
>> September 2003 post suggesting someone might be working on this, but
>> there was no followup that I could find.

> The quick workaround is to multiply the Greens with 2500/15 = 167 for the
> graphs. Use the orginal value for the text values below the graph.


Okay, I think I need to use CDEF to do this, altho I'm really confused by the examples given in the rrdgraph man page. I think the following statement will work:


Basically, jit = jitter * 167. Is there any way to do this WITHOUT reverse polish notation? I find that horribly confusing...or does someone have a good program to convert between "normal" math and RPN? If not, I'm sure I'll be bugging this list fairly often ;)

I tried the above and it seems to work great. My only remaining problem is that it seems that when samples are taken right before the clock tick that is used by rrdtool, and the next value is different, rrdtool records it as an average over time. This is a problem because jitter is always an integer...

i.e. at .90sec, it records a 3. At 1.40s, it records a 2. The value is apparently stored at 1.00s, so it writes down the value 2.89 or something similar. I tried using LAST as well as AVERAGE for the rrd creation, but I seem to get the same results. Is there a way to store results as integers or to round to integers before displaying?


Rob Nelson

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