Break mergeable up so that it's extensible

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+## Unreleased
+- Breaks up `mergeable(::ProcessedLayer)` into `mergeable(layer.plottype, layer.primary)` for dispatch purposes. This should enable `mergeable` to be extended by other packages that define recipes [#592](https://github.com/MakieOrg/AlgebraOfGraphics.jl/pull/592).
+
 ## v0.9.0 - 2025-01-30
 
 - **Breaking**: `paginate` now splits facet plots into pages _after_ fitting scales and not _before_ [#593](https://github.com/MakieOrg/AlgebraOfGraphics.jl/pull/593). This means that, e.g., categorical color mappings are consistent across pages where before each page could have a different mapping if some groups were not represented on a given page. This change also makes pagination work with the split X and Y scales feature enabled by version 0.8.14. `paginate`'s return type changes from `PaginatedLayers` to `Pagination` because no layers are stored in that type anymore. The interface to use `Pagination` with `draw` and other functions doesn't change compared to `PaginatedLayers`. `paginate` now also accepts an optional second positional argument which are the scales that are normally passed to `draw` when not paginating, but which must be available prior to pagination to fit all scales accordingly.
@@ -129,4 +132,4 @@
 - **Breaking**: Analyses now require parentheses (i.e. `linear()` instead of `linear`).
 - **Breaking**: Rename `layout_x` and `layout_y` to `col` and `row`.
 - **Breaking**: Rename `wts` keyword argument to `weights`.
-- **Breaking**: `categorical` has been replaced by `nonnumeric`.
+- **Breaking**: `categorical` has been replaced by `nonnumeric`.

src/algebra/layer.jl

@@ -425,18 +425,37 @@
 # Determine whether entries from a `ProcessedLayer` should be merged
 function mergeable(processedlayer::ProcessedLayer)
     plottype, primary = processedlayer.plottype, processedlayer.primary
-    # merge violins for correct renormalization
-    plottype <: Violin && return true
-    # merge stacked or dodged barplots
-    plottype <: Union{BarPlot,CrossBar} && return true
-    # merge waterfall plots
-    plottype <: Waterfall && return true
-    # merge dodged boxplots
-    plottype <: BoxPlot && haskey(primary, :dodge) && return true
-    # do not merge by default
+    return mergeable(plottype, primary)
+end
+
+# Default fallback implementation
+"""
+    mergeable(plottype::Type{<: Plot}, primary::Dictionaries.AbstractDictionary)::Bool
+
+Return whether the entries for the layer with `plottype` and `primary` should be merged.
+Merging means that all the data will be passed to a single plot call, instead of creating
+one plot object per scale.
+
+Return `true` if they **should** be merged, and `false` if **not** (the default).
+
+Extending packages should also extend this function on their own plot types 
+if they deem it necessary.  For example, beeswarm plots and violin plots
+need to be merged for correctness.
+"""
+function mergeable(plottype::Type{<: Plot}, primary)
     return false
 end
 
+# merge violins for correct renormalization
+mergeable(::Type{<: Violin}, primary) = true
+# merge stacked or dodged barplots
+mergeable(::Type{<: Union{BarPlot, CrossBar}}, primary) = true
+# merge waterfall plots
+mergeable(::Type{<: Waterfall}, primary) = true
+# merge dodged boxplots
+mergeable(::Type{<: BoxPlot}, primary) = haskey(primary, :dodge)
+
+
 # This method works on a list of "sliced" `ProcessedLayer`s
 function concatenate(pls::AbstractVector{ProcessedLayer})
     pl = first(pls)