Well, it looks like they vary by -+0.5kg, since this is what I'm doing here: http://www.stavros.io/misc/weight/
That's why I want a smoothing algorithm, but the one I found here[1] is giving me a bit of a problem (I can't see why the data points are being squeezed).
[1] http://www.swharden.com/blog/2008-11-17-linear-data-smoothin...