Allow vector (or column) of symbols in mapping

[ajinkya-k]

Problem description

if an aesthetic like color is mapped to a vector of symbols AoG throws an error, but if the symbols are converted to strings the mapping works. Here is an example (original example is from user jar on the julialang slack which i modified to use AoG instead of raw makie):

this code chunk throws an error:

let n = 50,  x = randn(n), y = randn(n), c = rand((:a, :b, :c), n)
    draw(
        mapping(x, y, color = c) * visual(Scatter),
        scales(Color = (; palette = from_continuous(Makie.wong_colors())))
    )
end

click triangle to see error

``` ERROR: MethodError: no method matching length(::Symbol) The function `length` exists, but no method is defined for this combination of argument types.

Closest candidates are:
length(::LaTeXStrings.LaTeXString)
@ LaTeXStrings ~/.julia/packages/LaTeXStrings/6NrIG/src/LaTeXStrings.jl:112
length(::Distributions.DirichletStats)
@ Distributions ~/.julia/packages/Distributions/fi8Qd/src/multivariate/dirichlet.jl:186
length(::VSCodeServer.JSON.Parser.MemoryParserState)
@ VSCodeServer ~/.vscode-oss/extensions/julialang.language-julia-1.127.2-universal/scripts/packages/JSON/src/Parser.jl:29
...

Stacktrace:
[1] length(g::Base.Generator{Symbol, typeof(identity)})
@ Base ./generator.jl:51
[2] _similar_shape(itr::Base.Generator{Symbol, typeof(identity)}, ::Base.HasLength)
@ Base ./array.jl:663
[3] collect(itr::Base.Generator{Symbol, typeof(identity)})
@ Base ./array.jl:790
[4] map(f::Function, A::Symbol)
@ Base ./abstractarray.jl:3399
[5] (::AlgebraOfGraphics.var"#195#200")(x::Tuple{Symbol})
@ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/algebra/processing.jl:85
[6] iterate
@ ./generator.jl:48 [inlined]
[7] collect(itr::Base.Generator{Base.Iterators.Zip{Tuple{Vector{Symbol}}}, AlgebraOfGraphics.var"#195#200"})
@ Base ./array.jl:791
[8] map(f::Function, A::Base.Iterators.Zip{Tuple{Vector{Symbol}}})
@ Base ./abstractarray.jl:3399
[9] getlabeledarray(layer::Layer, s::Vector{Symbol})
@ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/algebra/processing.jl:85
[10] (::AlgebraOfGraphics.var"#211#217"{Layer})(v::Vector{Symbol})
@ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/algebra/processing.jl:133
[11] map!(f::AlgebraOfGraphics.var"#211#217"{…}, out::Dictionaries.Dictionary{…}, d::Dictionaries.Dictionary{…})
@ Dictionaries ~/.julia/packages/Dictionaries/tq7Jt/src/map.jl:57
[12] map
@ ~/.julia/packages/Dictionaries/tq7Jt/src/map.jl:92 [inlined]
[13] process_mappings(layer::Layer)
@ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/algebra/processing.jl:133
[14] process(layer::Layer)
@ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/algebra/processing.jl:162
[15] ProcessedLayers
@ ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/algebra/layers.jl:68 [inlined]
[16] compute_axes_grid(d::Layer, scales::AlgebraOfGraphics.Scales; axis::Dictionaries.Dictionary{Symbol, Any})
@ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/algebra/layers.jl:263
[17] compute_axes_grid
@ ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/algebra/layers.jl:261 [inlined]
[18] _draw(d::Layer, scales::AlgebraOfGraphics.Scales; axis::Dictionaries.Dictionary{…}, figure::Dictionaries.Dictionary{…}, facet::Dictionaries.Dictionary{…}, legend::Dictionaries.Dictionary{…}, colorbar::Dictionaries.Dictionary{…})
@ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/draw.jl:157
[19] _draw
@ ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/draw.jl:154 [inlined]
[20] draw(d::Layer, scales::AlgebraOfGraphics.Scales; axis::@NamedTuple{}, figure::@NamedTuple{}, facet::@NamedTuple{}, legend::@NamedTuple{}, colorbar::@NamedTuple{}, palette::Nothing)
@ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/draw.jl:85
[21] draw(d::Layer, scales::AlgebraOfGraphics.Scales)
@ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/a4FNT/src/draw.jl:75
[22] top-level scope
@ Untitled-1:20

</details>

but the following works: 

let n = 50, x = randn(n), y = randn(n), c = rand((:a, :b, :c), n)
draw(
mapping(x, y, color = String.(c)) * visual(Scatter),
scales(Color = (; palette = from_continuous(Makie.wong_colors())))
)
end

[jkrumbiegel]

fixed by #651