[Rivet] Sigma-unit error bands

[David Grellscheid]

Hi all,

I just noticed that the error band plot can show units of sigma. 
However, this means that the label 'MC/Data' is now wrong, or am I 
getting something confused? Shouldn't it be something like

   \frac{ MC-Data }{ \Delta Data }

now?

See you,

   David