The subject of Time Series has lately been on my mind as well, see my blog posts on accuracy of Graphite vs RRD, as well as InfluxDB storage: http://grisha.org/
I am leaning towards none of the above being the best solution and am in the process of writing my own (too early to announce yet).
Like I mentioned here [0], whisper isn't the only storage option for graphite. Another user [1] mentioned blueflood. Have you evaluated any of these cassandra-based options?
At Foursquare, work was being done to put Hypertable under Graphite. It was incredible to use (my year+ queries returned in tens of milliseconds), but I don't know what came of it. Hypertable is criminally overlooked in the industry, and TSDB is a killer app; a little bit of glue code and you've basically invented a crude clone of Google's monitoring, with that stack.
I've looked at Cyanite, and it has the same accuracy problem as Graphite, which I describe in my blog post. Also I'm leaning towards the conclusion that something like Cassandra, which was design for Facebook scale is even a suitable store for something as compact as TS, which is just a stream of floats.
I am leaning towards none of the above being the best solution and am in the process of writing my own (too early to announce yet).