My apologies if I wasn't clear.

My intention was to say, with regards to graphing:

Graphing should be part of your overall tooling for monitoring boxes, but it should not be part of process monitoring. It should be a separate component (which should in turn be monitored by Nagios), for use in post-mortems and future planning.

It should not be built into Nagios, since the graph itself is completely irrelevent to whether Nagios should alert about a problem.

I also think that graphing is different from heueristic rate monitoring; graphs are for humans to spot trends/correlations. Alerting on rate changes is a binary alert/don't alert.

I.E. Cacti/Graphite should be creating graphs about services for human consumption, Nagios should be watching for problems with services.