GroveStreams looks like a pretty good idea; it took me an afternoon's messing around to understand how the "rollup" averaging system works, but after that it was straightforward. It looks as if you can update every 5-6 minutes (with several values in an update) and stay within their free limits, though there are other possible charges (eg. SMS) over a certain limit.
I used to use COSM and had just got it set up when Xively rolled in, and I couldn't be bothered with the absence of graphing facilities (which GroveStreams has in abundance).
All I can do now is hope that GroveStreams keeps running.
Will