diff --git a/docs/gallery/gallery/scales/custom_scales.jl b/docs/gallery/gallery/scales/custom_scales.jl
index 72ca0a20e..25d9323f3 100644
--- a/docs/gallery/gallery/scales/custom_scales.jl
+++ b/docs/gallery/gallery/scales/custom_scales.jl
@@ -42,15 +42,15 @@ plt = data(df) *
     visual(Arrows, arrowsize=10, lengthscale=0.3)
 draw(plt; palettes=(arrowcolor=colors, arrowhead=heads))
 
-# To associate specific attribute values to specific data values, use pairs.
-# Missing keys will cycle over values that are not pairs.
+# To associate specific attribute values to specific data values, use `sorter` or `renamer`
 
 x = rand(100)
 y = rand(100)
 z = rand(["a", "b", "c", "d"], 100)
 df = (; x, y, z)
-plt = data(df) * mapping(:x, :y, color=:z)
-colors = ["a" => colorant"#E24A33", "c" => colorant"#348ABD", colorant"#988ED5", colorant"#777777"]
+plt = data(df) * mapping(:x, :y, color=:z => sorter("a", "c"))
+# "a" will get color #E24A33 and "c" will get #348ABD
+colors = [colorant"#E24A33", colorant"#348ABD", colorant"#988ED5", colorant"#777777"]
 draw(plt; palettes=(color=colors,))
 
 # Categorical color gradients can also be passed to `palettes`.
diff --git a/docs/src/layers/mapping.md b/docs/src/layers/mapping.md
index dc74a1ad3..95da14ce2 100644
--- a/docs/src/layers/mapping.md
+++ b/docs/src/layers/mapping.md
@@ -82,3 +82,8 @@ The complete API of helper functions is available at [Mapping helpers](@ref).
 # column `labels` is expressed in strings and we do not want to treat it as categorical
 :labels => verbatim
 ```
+
+!!! note
+
+   Renaming/reordering helpers need not mention all values; e.g. `sorter(["medium",
+   "high"])` will order the unspecified value ("low") after "medimum" and "high".
diff --git a/src/algebra/layer.jl b/src/algebra/layer.jl
index 9e42f0aa6..2e69f3761 100644
--- a/src/algebra/layer.jl
+++ b/src/algebra/layer.jl
@@ -159,11 +159,17 @@ end
 function rescale(p::ProcessedLayer, categoricalscales::MixedArguments)
     primary = map(keys(p.primary), p.primary) do key, values
         scale = get(categoricalscales, key, nothing)
-        return rescale(values, scale)
+        result, scale = rescale(values, scale)
+        categoricalscales = set(categoricalscales, key => scale)
+        return result
     end
     positional = map(keys(p.positional), p.positional) do key, values
         scale = get(categoricalscales, key, nothing)
-        return rescale.(values, Ref(scale))
+        return map(values) do value
+            value, scale = rescale(value, scale)
+            categoricalscales = set(categoricalscales, key => scale)
+            return value
+        end
     end
 
     # compute dodging information
@@ -201,7 +207,7 @@ function concatenate(pls::AbstractVector{ProcessedLayer})
 end
 
 function append_processedlayers!(pls_grid, processedlayer::ProcessedLayer, categoricalscales::MixedArguments)
-    processedlayer = rescale(processedlayer, categoricalscales)
+    processedlayer, categoricalscales = rescale(processedlayer, categoricalscales)
     tmp_pls_grid = map(_ -> ProcessedLayer[], pls_grid)
     for c in CartesianIndices(shape(processedlayer))
         pl = slice(processedlayer, c)
diff --git a/src/algebra/layers.jl b/src/algebra/layers.jl
index d72800882..5120673aa 100644
--- a/src/algebra/layers.jl
+++ b/src/algebra/layers.jl
@@ -32,9 +32,9 @@ function compute_processedlayers_grid(processedlayers, categoricalscales)
     indices = CartesianIndices(compute_grid_positions(categoricalscales))
     pls_grid = map(_ -> ProcessedLayer[], indices)
     for processedlayer in processedlayers
-        append_processedlayers!(pls_grid, processedlayer, categoricalscales)
+        categoricalscales = append_processedlayers!(pls_grid, processedlayer, categoricalscales)
     end
-    return pls_grid
+    return pls_grid, categoricalscales
 end
 
 function compute_entries_continuousscales(pls_grid, categoricalscales)
@@ -109,7 +109,7 @@ function compute_axes_grid(s::OneOrMoreLayers;
     # fit categorical scales (compute plot values using all data values)
     map!(fitscale, categoricalscales, categoricalscales)
 
-    pls_grid = compute_processedlayers_grid(processedlayers, categoricalscales)
+    pls_grid, categoricalscales = compute_processedlayers_grid(processedlayers, categoricalscales)
     entries_grid, continuousscales_grid, merged_continuousscales =
         compute_entries_continuousscales(pls_grid, categoricalscales)
 
diff --git a/src/helpers.jl b/src/helpers.jl
index 0f06bb6d3..fa3fbf320 100644
--- a/src/helpers.jl
+++ b/src/helpers.jl
@@ -3,17 +3,34 @@
 to_string(s) = string(s)
 to_string(s::AbstractString) = s
 
-struct Sorted{T}
+struct Renamed{S, T}
     idx::UInt32
-    value::T
+    original::S
+    value::Union{T, Nothing}
+end
+Renamed(idx::Integer, original, value) = Renamed(convert(UInt32, idx), original, value)
+value(x::Renamed) = isnothing(x.value) ? x.original : s.value
+
+Base.hash(s::Renamed, h::UInt) = hash(value(s), hash(s.idx, hash(:Renamed, h)))
+function Base.:(==)(s1::Renamed, s2::Renamed) 
+    val_equal = isequal(s1.idx, s2.idx) && isequal(s1.value, s2.value) 
+    if val_equal && isnothing(s1.value)
+        return s1.original == s2.original
+    else
+        return val_equal
+    end
 end
-Sorted(idx::Integer, value) = Sorted(convert(UInt32, idx), value)
-
-Base.hash(s::Sorted, h::UInt) = hash(s.value, hash(s.idx, hash(:Sorted, h)))
-Base.:(==)(s1::Sorted, s2::Sorted) = isequal(s1.idx, s2.idx) && isequal(s1.value, s2.value)
 
-Base.print(io::IO, s::Sorted) = print(io, s.value)
-Base.isless(s1::Sorted, s2::Sorted) = isless((s1.idx, s1.value), (s2.idx, s2.value))
+Base.print(io::IO, s::Renamed) = print(io, value(s))
+function Base.isless(s1::Renamed, s2::Renamed)
+    if isnothing(s1.value) == isnothing(s1.value)
+        return isless((s1.idx, value(s1)), (s2.idx, value(s2)))
+    else
+        # sort any renamed values before values that keep
+        # their original value
+        return isless(!isnothing(s1.value), !isnothing(s2.value))
+    end
+end
 
 struct Renamer{U, L}
     uniquevalues::U
@@ -22,7 +39,7 @@ end
 
 function (r::Renamer{Nothing})(x)
     i = LinearIndices(r.labels)[x]
-    return Sorted(i, r.labels[i])
+    return Renamed(i, x, r.labels[i])
 end
 
 function (r::Renamer)(x)
@@ -30,7 +47,7 @@ function (r::Renamer)(x)
         cand = @inbounds r.uniquevalues[i]
         if isequal(cand, x)
             label = r.labels[i]
-            return Sorted(i, label)
+            return Renamed(i, x, label)
         end
     end
     throw(KeyError(x))
@@ -41,10 +58,11 @@ renamer(args::Pair...) = renamer(args)
 """
     renamer(arr::Union{AbstractArray, Tuple})
 
-Utility to rename a categorical variable, as in `renamer([value1 => label1, value2 => label2])`.
-The keys of all pairs should be all the unique values of the categorical variable and
-the values should be the corresponding labels. The order of `arr` is respected in
-the legend.
+Utility to rename a categorical variable, as in `renamer([value1 => label1, value2 =>
+label2])`. The order of `arr` is respected in the legend. The renamer need not specify all
+values of the sequence it renames: if the renamer is missing one of the values from a
+sequence it is applied to, the unspecified values are sorted after those that are specified (in the
+order returned by `unique`) and are not renamed.
 
 # Examples
 ```jldoctest
@@ -64,8 +82,8 @@ Class One
 ```
 
 If `arr` does not contain `Pair`s, elements of `arr` are assumed to be labels, and the
-unique values of the categorical variable are taken to be the indices of the array.
-This is particularly useful for `dims` mappings.
+unique values of the categorical variable are taken to be the indices of the array. This is
+particularly useful for `dims` mappings.
 
 # Examples
 ```jldoctest
@@ -90,10 +108,12 @@ end
 """
     sorter(ks)
 
-Utility to reorder a categorical variable, as in `sorter(["low", "medium", "high"])`.
-A vararg method `sorter("low", "medium", "high")` is also supported.
-`ks` should include all the unique values of the categorical variable.
-The order of `ks` is respected in the legend.
+Utility to reorder a categorical variable, as in `sorter(["low", "medium", "high"])`. A
+vararg method `sorter("low", "medium", "high")` is also supported. The order of `ks` is
+respected in the legend. The sorter need not specify all values (e.g. `sorter(["low",
+"medium"])` will work for an array that includes `"high"``); the unspecified values will be
+sorted after the specified values and will occur in the order returned by `unique`.
+
 """
 function sorter(ks::Union{AbstractArray, Tuple})
     vs = map(to_string, ks)
diff --git a/src/scales.jl b/src/scales.jl
index ecb8b3887..b99728fae 100644
--- a/src/scales.jl
+++ b/src/scales.jl
@@ -61,6 +61,9 @@ end
 datavalues(c::CategoricalScale) = c.data
 plotvalues(c::CategoricalScale) = c.plot
 getlabel(c::CategoricalScale) = something(c.label, "")
+function addvalue(c::CategoricalScale, values)
+    return CategoricalScale(vcat(c.data, values), c.plot, c.palette, c.label)
+end
 
 ## Continuous Scales
 
@@ -104,13 +107,14 @@ elementwise_rescale(value) = value
 
 contextfree_rescale(values) = map(elementwise_rescale, values)
 
-rescale(values, ::Nothing) = values
+rescale(values, ::Nothing) = values, nothing
 
 function rescale(values, c::CategoricalScale)
     # Do not rescale continuous data with categorical scale
     scientific_eltype(values) === categorical || return values
-    idxs = indexin(values, datavalues(c))
-    return plotvalues(c)[idxs]
+    scalevalues = vcat(datavalues(c), setdiff(values, datavalues(c)))
+    idxs = indexin(values, scalevalues)
+    return plotvalues(c)[idxs], addvalues(c, scalevalues)
 end
 
 Base.length(c::CategoricalScale) = length(datavalues(c))
diff --git a/test/helpers.jl b/test/helpers.jl
index 6f259c6a9..a9753c767 100644
--- a/test/helpers.jl
+++ b/test/helpers.jl
@@ -10,6 +10,8 @@
     @test r("a") < r("b") < r("c")
     @test r("a") == r("a")
     @test r("a") != r("b")
+    @test map(r, ["a", "a", "b", "c", "d"]) == [Sorted(1, "A"), Sorted(1, "A"), Sorted(2, "B"), Sorted(3, "C"), Sorted(4, "d")]
+
     r̂ = renamer(["a" => "A", "b" => "B", "c" => "C"])
     @test r̂("a") == Sorted(1, "A")
     @test r̂("b") == Sorted(2, "B")
@@ -33,6 +35,8 @@
     @test s("b") < s("c") < s("a")
     @test s("a") == s("a")
     @test s("a") != s("b")
+    @test map(s, ["a", "b", "c", "d"]) == [Sorted(3, "a"), Sorted(1, "b"), Sorted(2, "c"), Sorted(4, "d")]
+    
     ŝ = sorter(["b", "c", "a"])
     @test ŝ("a") == Sorted(3, "a")
     @test ŝ("b") == Sorted(1, "b")