[latchkey]

Not sure why you got voted down, so I'll vote you back up and answer your totally valid question.

rrdtool is fine if you know your schema in advance, but that is somewhat of a hinderance because you don't always know it or want to have to define it.

Graphite is great because it listens on a port an accepts data in a simple unstructured format. The format is: key : value (long) : timestamp

Where key can be in dot notation so that you can have your data in a 'tree' like structure. This is a made up example: "ehcache.activemq.joins" or "ehcache.activemq.quits". The dashboard can then give you a nice tree to navigate through your data so that you can pin down specific areas that you'd like to graph. It also allows you to easily mix and match areas as overlays.

Hope that explains things.