This functionality requires Graphite 1.0 to be
compiled. The current stable release does not have the
function groupByNode. This feature will be available
when Graphite 1.0 is released.

Graphite 1.0 Functions
[http://graphite.readthedocs.org/en/1.0/functions.html]

The callback aggregator is a function will be applied
to the result of groups returned. In this example the
group will be the hard disk name (Ex: sda1). The 
function "sumSeries" will be applied to each result 
adding the total read iops of all like named disks
and graphing the result.