Not OP, but the API looks slightly dense/shorter/brief, which might appeal some people. I use Altair too, and initially felt the API very verbose compared to,

Not OP, but the API looks slightly dense/shorter/brief, which might appeal some people. I use Altair too, and initially felt the API very verbose compared to, say, matplotlib (for simpler plots). So I kind of see the possible appeal. But I now appreciate the grammar-of-graphics like composable nature of Altair API. And yeah, the interactivity is nice.