facet_chart: opt-in linked zoom/pan/selection across panels (default stays independent)

[Darksinian]

Summary

Feature request: facet panels should support linked interactions — synchronized zoom/pan (and optionally box-selection) across panels — behind an explicit opt-in. The default should remain what it is today: every panel zooms, pans, and selects independently.

Current behavior

facet_chart(share_x=True, share_y=True) only aligns the panels' initial domains at build time (components.py:3209 computes shared dims once). At runtime each panel is an independent renderStandalone instance (facets.py:170), so:

• zooming/panning one panel leaves the others at their original view — comparing the same region across panels means manually reproducing the same box zoom N times;
• box-select is per-panel; there's no way to brush one panel and see the same x-range (or the selected rows) highlighted in the rest.

There is no API to change this.

Proposed API

xy.facet_chart(
    xy.scatter(x="x", y="y"),
    by="g",
    cols=2,
    data=df,
    link="x",        # None (default) | "x" | "y" | "both"
    link_select=True # default False: brushing one panel echoes the selection to all
)

• link=None (default): today's behavior, fully independent panels.
• link="x" | "y" | "both": a zoom/pan in any panel broadcasts its new domain to the other panels on the linked dimension(s). Natural pairing with share_x/share_y — linking a dimension probably implies sharing its initial domain.
• link_select (or fold into link): a brush in one panel applies the same data-space predicate across panels, and on_select fires once with the union.

Notes

• Double-click reset should reset all linked panels together.
• The render client already recomputes per-view domains on zoom; linking is a broadcast of the view window between panel instances in the same grid (all panels live in one document/widget, so no kernel round-trip is required for the view sync itself).
• Related: FacetChart notebook display leaks global CSS into the page (no widget path; _repr_html_ returns a standalone document) #90 (facet display path) — if facets grow a proper widget path, the link channel could ride the same container.

Environment/context: xy main @ 541a7da, playground testing in Jupyter.