[Rivet] make-plots (effectively) doesn't show dashed lines in --mc-errs mode

[Frank Siegert]

Hi Andy,

> PS. On thinking about it, it's a bit inconsistent that compare-histos
> shows a continuous line by default but rivet-mkhtml uses the mc-errs
> setting by default. Any opposition to making rivet-mkhtml's default
> behaviour the same as the compare-histos? I presume that Frank likes it
> that way, since he wrote it, but there are a few reasons why it would be
> more convenient for me if web pages made by other people with the
> default settings made output with the continuous line style that's
> suitable for high stats runs as presented in talks and papers!

I don't have a strong opinion either way, so feel free to change that 
default.

Cheers,
Frank