Support title/subtitle/caption in vgplot

[frtennis1]

Observable Plot has the ability to specify a title, subtitle, and/or caption on a plot https://observablehq.com/plot/features/plots#other-options.

It would be nice to pass through these options in vgplot so Mosaic can use them as well.

[frtennis1]

@jheer hiya! If you get a sec, could I get your thoughts on the pull request? It's a pretty small (and hopefully uncontroversial) change.

[jheer]

Thanks for your patience (and for the PR!). I still need to think through this a bit more, as I'm not yet 100% sure we want to expose this. For example, we might instead consider adding title, caption, etc to the spec language as new components which could work in any Mosaic spec (and not just Plot) in a consistent way.

And if we do choose to expose these I need to do some testing to make sure that any HTML wrappers that Plot adds to titled/captioned plots don't interfere with our assumptions when adding interactors to the generated content.

[frtennis1]

Aha, gotcha. Does seem more involved than I initially thought. Let me know if I can help with any of those steps! (e.g. if you become convinced adding title/caption/spec as new components rather than top-level attributes is the way to go, I can look into changing the PR to do it that way whenever I get a moment if you'd like)

[domoritz]

If we want to try that route, I'd recommend a new pull request so we can compare the results better.