I want to mark some events on my Munin graphs to see how the graphs change at these points. It seems that in RRD terms they can be created by VRULE command, but I can't find a way to use it with Munin. Essentially I want to pass a list of time:caption or at least just time values and ideally see the corresponding lines on all graphs.
if this is not possible with Munin, is there a similar monitoring system (with custom plugin support) that allows it?
Munin (and many of the other Linux based monitoring products) use rrdtool in the background as you've seen. The benefit of that is you can build your own graphs really easily outside of the tool. I suggest you just write a shell script which creates a couple of graphs you need with the relevant VRULE entries. The advantage is you'll be able to combine any data you want over any time period (assuming it's still present). You could either just output images into a directory and browse to them or you could wrap some html / php around them.
I view Munin (and similar products, although I prefer Munin) as primarily a set of data collection scripts, with a default front end. Munin can do some customisation and consolidation but since it's collected the data, and since the interface to rrdtool is pretty simple, I prefer just to create as many custom graphs as I want on an adhoc basis.
