Bad error message when forgetting to set data

[jkrumbiegel]

This happens easily when building more complex expressions and forgetting parentheses, like data * layer1 + layer2 instead of data * (layer1 + layer2).

julia> mapping(:x, :y) * visual(Scatter) |> draw
ERROR: MethodError: no method matching length(::Symbol)

Closest candidates are:
  length(::DataStructures.EnumerateAll)
   @ DataStructures ~/.julia/packages/DataStructures/95DJa/src/multi_dict.jl:96
  length(::DelaunayTriangulation.BoundingInterval)
   @ DelaunayTriangulation ~/.julia/packages/DelaunayTriangulation/74rOt/src/data_structures/trees/rtree.jl:62
  length(::TranscodingStreams.Buffer)
   @ TranscodingStreams ~/.julia/packages/TranscodingStreams/O3BYF/src/buffer.jl:43
  ...

Stacktrace:
  [1] length(g::Base.Generator{Symbol, typeof(identity)})
    @ Base ./generator.jl:50
  [2] _similar_shape(itr::Base.Generator{Symbol, typeof(identity)}, ::Base.HasLength)
    @ Base ./array.jl:710
  [3] collect(itr::Base.Generator{Symbol, typeof(identity)})
    @ Base ./array.jl:833
  [4] map(f::Function, A::Symbol)
    @ Base ./abstractarray.jl:3313
  [5] (::AlgebraOfGraphics.var"#190#195")(x::Tuple{Symbol})
    @ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/algebra/processing.jl:85
  [6] iterate
    @ ./generator.jl:47 [inlined]
  [7] collect(itr::Base.Generator{Base.Iterators.Zip{Tuple{Vector{Symbol}}}, AlgebraOfGraphics.var"#190#195"})
    @ Base ./array.jl:834
  [8] map(f::Function, A::Base.Iterators.Zip{Tuple{Vector{Symbol}}})
    @ Base ./abstractarray.jl:3313
  [9] getlabeledarray(layer::Layer, s::Symbol)
    @ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/algebra/processing.jl:85
 [10] (::AlgebraOfGraphics.var"#204#210"{Layer})(v::Symbol)
    @ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/algebra/processing.jl:128
 [11] iterate
    @ ./generator.jl:47 [inlined]
 [12] _collect(c::Vector{…}, itr::Base.Generator{…}, ::Base.EltypeUnknown, isz::Base.HasShape{…})
    @ Base ./array.jl:854
 [13] collect_similar
    @ ./array.jl:763 [inlined]
 [14] map
    @ ./abstractarray.jl:3285 [inlined]
 [15] process_mappings(layer::Layer)
    @ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/algebra/processing.jl:128
 [16] process(layer::Layer)
    @ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/algebra/processing.jl:162
 [17] ProcessedLayers
    @ ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/algebra/layers.jl:55 [inlined]
 [18] compute_axes_grid(d::Layer, scales::AlgebraOfGraphics.Scales; axis::Dictionaries.Dictionary{Symbol, Any})
    @ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/algebra/layers.jl:250
 [19] compute_axes_grid
    @ ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/algebra/layers.jl:248 [inlined]
 [20] compute_axes_grid(fig::Figure, d::Layer, scales::AlgebraOfGraphics.Scales; axis::Dictionaries.Dictionary{Symbol, Any})
    @ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/algebra/layers.jl:218
 [21] compute_axes_grid
    @ ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/algebra/layers.jl:215 [inlined]
 [22] #363
    @ ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/draw.jl:20 [inlined]
 [23] update
    @ ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/draw.jl:10 [inlined]
 [24] plot!(fig::Figure, d::Layer, scales::AlgebraOfGraphics.Scales; axis::Dictionaries.Dictionary{Symbol, Any})
    @ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/draw.jl:20
 [25] plot!
    @ ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/draw.jl:16 [inlined]
 [26] (::AlgebraOfGraphics.var"#374#379"{…})(f::Figure)
    @ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/draw.jl:162
 [27] update
    @ ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/draw.jl:10 [inlined]
 [28] _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/OTZgv/src/draw.jl:161
 [29] _draw
    @ ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/draw.jl:154 [inlined]
 [30] draw(d::Layer, scales::AlgebraOfGraphics.Scales; axis::@NamedTuple{}, figure::@NamedTuple{}, facet::@NamedTuple{}, legend::@NamedTuple{}, colorbar::@NamedTuple{}, palette::Nothing)
    @ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/draw.jl:85
 [31] draw
    @ ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/draw.jl:75 [inlined]
 [32] draw(d::Layer)
    @ AlgebraOfGraphics ~/.julia/packages/AlgebraOfGraphics/OTZgv/src/draw.jl:75
 [33] |>(x::Layer, f::typeof(draw))
    @ Base ./operators.jl:917
 [34] top-level scope
    @ REPL[35]:1