jasondavies/d3 — comment on commit ebab983b8a644b4e5a7fe3d392be67d44c3e900b
File: src/core/interpolate.js line 196
By mbostock:

Hmm… I think `c` would have to be initialized as a `d3.map()`, too, if you want to be able to interpolate the __proto__ property? But I guess that's a pretty bogus use case, because I can't think of any situation where it would actually make sense to do so.