[rrd-developers] Historical comparison graphs (feature for 1.1.x?)

[Terminator rAT]

Hi, folks.

We primarily use RRDtool to generate "usage" graphs for our network
customers.  One feature that we'd like to see in RRDtool is a way to
compare current data with historical data within the same graph.

For example, imagine you're graphing network usage (or natural gas or
whatever).  Obviously you want to display the usage for this month.  But
it would be nice to overlay this with your "this month last year" usage,
to give an idea of trending.

There doesn't appear to be any facility in RRDtool for graphing differing
time periods, so I propose the following additional data "type":

	ODEF (Offset DEFinition)
		ODEF:var=rrdfile:ds:CF:offset_time

For the example above (this month and this month last year), you would do
something like this (excuse the pseudo-code):

	rrdtool graph usage_comparison.gif --start -1month ... \
		DEF:usage:data.rrd:netusage:MAX \
		ODEF:lastyear:data.rrd:netusage:MAX:-1y \  <-- note offset
		AREA:usage#00cc00:Network Utilization \
		LINE1:lastyear#000000:Utilization This Month Last Year

I haven't looked much into how to implement this functionality, but I'd
love to see it in one of the upcoming releases.  If anyone has done this,
or could point me in the right direction to code it myself, I'd be happy
to help integrate it.

Thanks!

	-rAT

-- 
Karl Schilke / kschilke at eli.net / (360)816-3837 / FAX: (360)816-3522
Senior Software Engineer, Network Management Systems && Applications
Electric Lightwave, Inc. / 4400 NE 77th Avenue / Vancouver, WA 98662


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