Extensions for vgplot

[domoritz]

People may want to add new clients like charts or widgets to vgplot or even override the built-in clients like the table. It would be nice if there was a way to register new clients that can be used in the spec.

A possible API would be to register a new client could be this:

import { registerClient } from "@uwdata/vgplot";

registerClient("foo", CustomClient);

{
  "hconcat: [
    { "foo": { x: "x", y: "y", table: "table", filter: "$filter" } }
  ]
}

One tricky thing will be to provide some way to extend the TS types for spec with a new client.

[jheer]

The vgplot API already supports extensions and overrides. One can use new clients directly by creating their own directive methods, and they can add them to a vg API instance by passing them to the createAPIContext method.

The spec parser is also extensible, as one can specify component parsers via ParseContext's component option. Still, one needs to provide a proper parse function that returns compatible AST nodes.

That said, we need better documentation for this, maybe as part of #394.

How best to extend the TS types remains an open question.