Have you thought about making the name of the property xSort (and ySort)? That seems to be more consistent with the existing camel-case names like groupX and barY. (The functionality is a really welcome addition, by the way, particularly if allowing Plot to handle it is more efficient than using d3.groupSort externally.

Thanks for the suggestion. In fact this build was late wrt to the repo—the current version in the branch uses sortX/sortY. I've just updated it here. It's quite possible that it might be a bit more efficient for huge datasets, in the sense that it saves one aggregation loop—but the intent is primarily to enhance expressivity.