Internals

AlgebraOfGraphics is based on contexts, which can be extended. A context, together with a named tuple (used for x, y axes or attributes in the plot) forms a Mapping. Each Mapping can actually encode many traces, encoded as a list of NamedTuple => Mapping pairs, accessible using pairs(st::Mapping).

Contexts

The DataContext is invoked with data(df), where df respects the Tables.jl interface.

using RDatasets: dataset
using AlgebraOfGraphics
mpg = dataset("ggplot2", "mpg")
t = data(mpg)
st = mapping(:Cyl, color = :Year => categorical)

pairs(t * st)

2-element Array{Pair{NamedTuple{(:color,),Tuple{NamedDims.NamedDimsArray{(:Year,),CategoricalArrays.CategoricalValue{Int32,UInt32},1,CategoricalArrays.CategoricalArray{Int32,1,UInt32,Int32,CategoricalArrays.CategoricalValue{Int32,UInt32},Union{}}}}},AlgebraOfGraphics.Mapping},1}:
 (color = [1999],) => Mapping with entries (Symbol("1"),)
 (color = [2008],) => Mapping with entries (Symbol("1"),)

In the dims(i) context, all variables in mapping are sliced along the i-th dimension. The resulting arrays of arrays broadcasted to a common shape, and each "inner array" corresponds to a separate trace. The syntax dims exists to allow setting discrete attributes variables that only vary with one of the dimensions.

d = dims(1) * mapping(rand(5, 3, 2), rand(5, 3), color = dims(2))
pairs(d)

6-element Array{Pair{NamedTuple{(:color,),Tuple{Int64}},AlgebraOfGraphics.Mapping},1}:
 (color = 1,) => Mapping with entries (Symbol("1"), Symbol("2"))
 (color = 2,) => Mapping with entries (Symbol("1"), Symbol("2"))
 (color = 3,) => Mapping with entries (Symbol("1"), Symbol("2"))
 (color = 1,) => Mapping with entries (Symbol("1"), Symbol("2"))
 (color = 2,) => Mapping with entries (Symbol("1"), Symbol("2"))
 (color = 3,) => Mapping with entries (Symbol("1"), Symbol("2"))

Combining mappings and contexts

All outputs of mapping, data, and dims inherit can be combined using + (adding a new layer), or * (merge information in existing layer).

using CairoMakie
mpg1 = copy(mpg)
mpg1.Displ = mpg.Displ .* 0.1
ts = data(mpg) * visual(markersize = 5) + data(mpg1) * visual(markersize=10)
sl = ts * mapping(:Hwy, :Displ, color = :Cyl => categorical)

AlgebraOfGraphics.AlgebraicList{AlgebraOfGraphics.Spec{Any}}(AlgebraOfGraphics.Spec{Any}[AlgebraOfGraphics.Spec{Any}((), NamedTuple(), Mapping with entries (Symbol("1"), Symbol("2")), (markersize = 5,)), AlgebraOfGraphics.Spec{Any}((), NamedTuple(), Mapping with entries (Symbol("1"), Symbol("2")), (markersize = 10,))])

The result can then be plotted using the draw function:

sl * visual(Scatter) |> draw

Implementing a new context

To implement a new context, one can overload:

AlgebraOfGraphics._merge(c::MyContext, s1::Mapping, s2::Mapping), to allow applying mapping to MyContext,
AlgebraOfGraphics._pairs(c::MyContext, s::Mapping), which must iterates group::NamedTuple => mapping::Mapping pairs.

See example implementation in the context file.

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