add highlight transformation

src/AlgebraOfGraphics.jl

@@ -33,7 +33,7 @@ export hideinnerdecorations!, deleteemptyaxes!
 export Layer, Layers, ProcessedLayer, ProcessedLayers
 export Entry, AxisEntries
 export renamer, sorter, nonnumeric, verbatim, presorted
-export density, histogram, linear, smooth, expectation, frequency, contours, filled_contours
+export density, histogram, linear, smooth, expectation, frequency, contours, filled_contours, highlight
 export visual, data, geodata, dims, mapping
 export datetimeticks
 export draw, draw!
@@ -69,6 +69,7 @@ include("transformations/frequency.jl")
 include("transformations/expectation.jl")
 include("transformations/contours.jl")
 include("transformations/filled_contours.jl")
+include("transformations/highlight.jl")
 include("guides/guides.jl")
 include("guides/legend.jl")
 include("guides/colorbar.jl")

src/algebra/layer.jl

@@ -169,8 +169,9 @@ end
 
 unnest(vs::AbstractArray, indices) = map(k -> [el[k] for el in vs], indices)
 
-unnest_arrays(vs) = unnest(vs, keys(first(vs)))
+unnest_arrays(vs) = isempty(vs) ? [[]] : unnest(vs, keys(first(vs)))
 function unnest_dictionaries(vs)
+    isempty(vs) && return Dictionary()
     return Dictionary(Dict((k => [el[k] for el in vs] for k in collect(keys(first(vs))))))
 end
 slice(v, c) = map(el -> getnewindex(el, c), v)
@@ -192,6 +193,21 @@ function Base.map(f, processedlayer::ProcessedLayer)
     return ProcessedLayer(processedlayer; positional, named)
 end
 
+function filtermap(f, processedlayer::ProcessedLayer)
+    axs = shape(processedlayer)
+    outputs = map(CartesianIndices(axs)) do c
+        return f(slice(processedlayer.positional, c), slice(processedlayer.named, c))
+    end
+    to_remove = outputs .== nothing
+    deleteat!(outputs, to_remove)
+    primary = map(processedlayer.primary) do value
+        value[.!to_remove]
+    end
+
+    positional, named = unnest_arrays(map(first, outputs)), unnest_dictionaries(map(last, outputs))
+    return ProcessedLayer(processedlayer; positional, named, primary)
+end
+
 ## Get scales from a `ProcessedLayer`
 
 function uniquevalues(v::AbstractArray)

src/guides/legend.jl

@@ -92,6 +92,9 @@ end
 
 categorical_scales_mergeable(c1, c2) = false # there can be continuous scales in the mix, like markersize
 
+is_empty_categorical_scale(s::CategoricalScale) = isempty(datavalues(s))
+is_empty_categorical_scale(s::ContinuousScale) = false
+
 function compute_legend(grid::Matrix{AxisEntries}; order::Union{Nothing,AbstractVector})
     # gather valid named scales
     scales_categorical = legendable_scales(Val(:categorical), first(grid).categoricalscales)
@@ -100,7 +103,7 @@ function compute_legend(grid::Matrix{AxisEntries}; order::Union{Nothing,Abstract
     scales = Iterators.flatten((pairs(scales_categorical), pairs(scales_continuous)))
 
     # if no legendable scale is present, return nothing
-    isempty(scales) && return nothing
+    isempty(scales) || all(x -> all(is_empty_categorical_scale, last(x)), scales) && return nothing
 
     scales_by_symbol = Dictionary{Symbol,ScaleWithMeta}()
 

src/transformations/highlight.jl

@@ -0,0 +1,51 @@
+struct Highlight{F<:Function}
+    target::Vector{Union{Int,Symbol}}
+    predicate::F
+    repeat_facets::Bool
+end
+
+function Highlight(
+    target::Vector,
+    predicate;
+    repeat_facets = false
+)
+    return Highlight(
+        convert(Vector{Union{Int,Symbol}}, target),
+        predicate,
+        repeat_facets
+    )
+end
+
+function (highlight::Highlight)(p::ProcessedLayer)
+    primary = AlgebraOfGraphics.dictionary([(key == :color ? :group : key, value) for (key, value) in pairs(p.primary)
+        # should the data be repeated across all facets or not? maybe needs to be an option
+        if !(highlight.repeat_facets && key in (:layout, :row, :col))
+    ])
+
+    grayed_out = ProcessedLayer(p; primary, attributes = merge(p.attributes, AlgebraOfGraphics.dictionary([:color => :gray80])))
+
+    function apply_predicate(target::Vector, predicate::Function, positional, named, scalar_primaries)
+        b = predicate([resolve_target(t, positional, named, scalar_primaries) for t in target]...)
+        b isa Bool || error("Highlighting predicate returned non-boolean value $b")
+        return b
+    end
+
+    resolve_target(target::Int, positional, named, scalar_primaries) = positional[target]
+    resolve_target(target::Symbol, positional, named, scalar_primaries) = haskey(scalar_primaries, target) ? scalar_primaries[target] : named[target]
+
+
+    i::Int = 0
+    colored = AlgebraOfGraphics.filtermap(p) do positional, named
+        i += 1
+        scalar_primaries = map(val -> val[i], p.primary)
+        if apply_predicate(highlight.target, highlight.predicate, positional, named, scalar_primaries)
+            return positional, named
+        else
+            return nothing
+        end
+    end
+
+    return ProcessedLayers([grayed_out, colored])
+end
+
+highlight(pair::Pair; kwargs...) = AlgebraOfGraphics.transformation(Highlight(vcat(pair[1]), pair[2]; kwargs...))