[lo5]

> With complex marks it is ambiguous what is a data point and what is a series.

I sort of agree with you. I've implemented the Grammar of Graphics from the ground up four times professionally (!), twice in collaboration with Leland, all in different products.

The main reason why it might be finicky to work with directly is that the point vs. series vs. series-of-series distinction can run arbitrarily deep, so there's some mental gymnastics involved on the part of the library's user how to refactor the data and present it correctly so that the library can do its thing.

Tableau, which is also a GoG system, sort of deals with this by having slots for "Dimensions", "Pages", "Color", etc. as proxies for multi-level aggregation ("group/slice/dice" in BI terms). So even though it's not immediately apparent to new users how to present data correctly to the rendering system to get the kind of vis they want, at least it's pretty low-friction UX to shuffle variables between those slots till satisfied.

With programmatic use, that shuffling-around gets cumbersome because now you have write code to munge data into submission.

Tableau introduced the "Show Me" feature precisely for this reason - most new users would rather get stuff done quickly than figure out how the GoG can best solve their vis problem.