Pup will graph all the metrics Datadog does - it already collects OS metrics, and comes with a set of plug-ins for connecting to common apps, and it wouldn't be very difficult to connect collectd to it.

Pup's feature-set is limited today, though, compared to graphite. You may also check-out the full http://datadoghq.com service for more. It's SaaS, and free up to 5 servers.