theme gains a new element panel.merge to control whether to merge anel area into single gTree

[Yunuuuu]

Try to fix #6396

[Yunuuuu]

I'm not sure whether the current implementation will be accepted. I've reviewed the tests—while some changes are present, there don’t appear to be any visual differences. The current error is due to the vignette, which I plan to tweak once the implementation itself is accepted.

[teunbrand]

Hi Yunuuuu,

Thanks for putting together the PR to fix the issue, however we feel that the added complexity of the code does not result in sufficient utility.
Moreover, several reverse dependencies (like patchwork) may also rely on the current panel structure. While I understand why you'd want a panel spanning dendrogram, I do not think viewport-based tricks will be the cleanest path to achieve what you want. Instead, I'll suggest that you implement a Facet subclass where you have control over placing panels and annotations. Moreover, because facets have access to scales, you should be able to derive the break locations of discrete scales, so you don't have to rely on these viewports.
Because we feel a superior alternative is available and we're reluctant to add this level of complexity for a niche feature, we're closing this PR.

[Yunuuuu]

After testing, I found it difficult to handle this using a single Facet object alone, since the panels input to Facet$draw_panels() is unpredictable. It’s challenging to break down panels into individual layer grobs.

Several issues contribute to this unpredictability:

• The Layer$draw_geom method doesn't always return a grob if the developer doesn’t follow the expected conventions. No error or warning is issued in such cases, and gList(NULL) or gList(gList()) simply works silently.
• Coord$draw_panel will sometimes wrap all layer grobs into single gTree in CoordRadial, sometimes it won't

Solutions:

• Ensure that Layer$draw_geom() always returns a single grob or a gTree.
• always wrap the all layer grobs into a single gTree before passing to ``Coord$draw_panel`