[Rivet] Sigma-unit error bands

[David Grellscheid]

> I thought that when switched to (IIRC) RatioPlotStyle=deviation, the
> default ratio y label became "Deviation". Is that not happening?

See e.g.

http://www.ippp.dur.ac.uk/~richardn/qed/plots4/D0_2010_S8570965/d01-x01-y01.png

   David