Have you considered opentsdb or graphite? I love graphite because of the nice frontend interface and functionality it provides for visualizing and transforming your metrics.
For data storage Cyanite [0] speaks the graphite protocol and stores the data in Cassandra. Alternately, InfluxDB [1] speaks the graphite protocol and stores the data in itself
To get the data back out, there's graphite-api [2] which can be hooked up to cyanite [3] or influxdb [4]. You can then connect any graphite dashboard you like, such as grafana [5], to it.
[0] https://github.com/pyr/cyanite [1] http://influxdb.com [2] https://github.com/brutasse/graphite-api [3] https://github.com/brutasse/graphite-cyanite [4] https://github.com/vimeo/graphite-influxdb [5] http://grafana.org
Last I looked at Graphite I balked at the data store design (very I/O heavy) and the awful front ends (very limited graphing and reporting capabilities). But I haven't discovered a good alternative that has traciton. Diamond seems like the thing to use for collecting metrics (instead of collectd), though.
Edit: Grafana looks good, actually.