missing channel error message is sometimes misleading

[Fil]

no big deal, but I don't know how to fix it

[mbostock]

Took a brief look at fixing this. Preferably the error should indicate all required channels that are missing, not just the first one that’s missing. We could use a special subclass of Error in Mark when this happens, and then the convenience aliases such as rectY could catch that error and replace it with a message that is appropriate to the alias.

[Fil]

Ref. #117

[mbostock]

This was fixed in #348 when we added implicit stacking.