The group transform should generate empty groups for extra values in the domain?

[mbostock]

Currently, the domain option can only be used to generate a subset of groups, but I think we also want to generate empty groups for any other values in the domain, and also we should respect the specified order of the domain rather than always sorting naturally.

[mbostock]

The domain option was removed in #272; we’re going to set the scale’s domain instead, and then respect that in the group transform.

[mbostock]

Er, we won’t respect it the group transform, I guess (because that happens prior to the scale’s domains being computed), but the resulting marks won’t be rendered when #52 is fixed.