Make it easier to compare different benchmarks

When comparing multiple different benchmarks, it would be nice to have a way to mark them as "comparable" (perhaps `bcomparisongroup`), with the following effects:

* numbers are all generated on the same time scale (e.g. all 'μs' or all 'ms', not a mixture)
* graphs are produced with the same vertical scale

This would make it easier to eyeball differences.

As an extension, perhaps also the library could do some statistics to see if there are true differences between the timings (ANOVA? it's been a while...), but that would be an added bonus.