ganglia-style graph aggregation with hobbit

[Gildas Le Nadan <gn1 (at) sanger.ac.uk>]

Hello,

I'd like to completely replace the ganglia system we have here with hobbit.

Most of the needed features are there, except the ability to produce 
aggregated graphs with multiple hosts, as in 
http://monitor.millennium.berkeley.edu/?c=PSI%20Cluster&m=&r=hour&s=descending&hc=4

I suppose the following things will need to be done:
- find a neat way to group the hosts (all the hosts in a group or a page 
for instance)
- collect all the values from the rrd for all the hosts in a group and 
add them (when different from 0/NaN)
- graph the result

What would be the best way to do it? I'd like not to reinvent the wheel, 
so if some bits are already existing, I'd better use them...

Cheers,
Gildas