http://blueflood.io/ is an option. It's built on top of cassandra and has experimental support for use as a backend for graphite-web. There are several engineers still actively working on it who are generally happy to help with any issues raised via irc or the mailing list. Unsure what you mean by 'massive', but I've used it to store billions of data points per day successfully.

Disclaimer: I'm a former core contributor to blueflood.