Thanks for your comments!

IMO, the grammar level is when it starts to feel like a locked ecosystem. Both D3 and vega let you compose charts from low-level building blocks but D3 is much more open and you can use only part of it or combine it with other libraries with little effort. Vega on the other hand is more difficult to bring in other libraries and easier to use by itself alone, but it alone already can do the majority of what people want.

The line between grammar and high-level are more blurred. High-level in my definition means when there are references of non-primitive chart types by name (e.g. candlestick, etc.). Many libraries provide both level of capabilities.

The content in my talk at OutlierConf might be a bit more updated.

https://twitter.com/OutlierConf/status/1392466337537028105?s=20