feat: use columnar data within vgplot

[domoritz]

This defers chasing the arrow tables to arrays of objects and optimizes some of the computation to use columnar processing (e.g. in nearest).

There are possibly more things to optimize (e.g. Plot itself observablehq/plot#191) and we can remove some of the special handling when Arrow supports Decimals better.

[domoritz]

Hmm, this breaks the observable latency example. Need to check what's up.

[jheer]

I like the goal here, but some of these changes probably won’t help much (and others will hurt performance) until Plot also supports optimized columnar data.

Using Arrow’s automatic row object generation (via Proxy) has a non-trivial overhead cost. That’s why I have our own faster object generation method (arrowToObjects) to avoid costly unpacking within Plot. So for now we should still use that even if no type conversions are needed.

[mbostock]

Just chiming in here — Plot already supports optimized columnar data, it just doesn’t support shorthand for that data. I added an example to observablehq/plot#191 (comment) that shows how to pass in columnar data. Please let me know if we can help!

[domoritz]

Just chiming in here — Plot already supports optimized columnar data, it just doesn’t support shorthand for that data. I added an example to observablehq/plot#191 (comment) that shows how to pass in columnar data. Please let me know if we can help!

Something like 62e72fa (#334) ?

[mbostock]

That looks right to me, @domoritz! 👍