This is surprising! I would have imagined only 3 items returned, but instead they're always 0 for y1 and y2....

select doesn't touch the channels, but modifies the facets indices instead

Interesting! At some point I'd be curious about that API design choice.

that is the dataframe way; see for example https://pandas.pydata.org/docs/getting_started/intro_tutorials/03_subset_data.html

it's plotting x and x/basis(z), so you get a line (that goes through the origin [0,0]) for each group z