Cant plot empty vectors of LineString

[rafaqz]

It would be good if this worked so we don't have to plot dummy linestrings

julia> Makie.plot(LineString[])
ERROR: BoundsError: attempt to access 0-element Vector{LineString} at index [1]
Stacktrace:
  [1] getindex
    @ ./essentials.jl:13 [inlined]
  [2] convert_arguments(PB::PointBased, linestring::Vector{LineString})
    @ Makie ~/.julia/dev/Makie/src/conversions.jl:214
  [3] convert_arguments(T::Type{Lines{Tuple{Vector{LineString}}}}, args::Vector{LineString}; kw::Base.Pairs{Sym
ol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Makie ~/.julia/dev/Makie/src/conversions.jl:10
  [4] convert_arguments
    @ ~/.julia/dev/Makie/src/conversions.jl:7 [inlined]
  [5] plot!(scene::Scene, P::Type{Any}, attributes::Attributes, args::Vector{LineString}; kw_attributes::Base.P
irs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Makie ~/.julia/dev/Makie/src/interfaces.jl:317
  [6] plot!(scene::Scene, P::Type{Any}, attributes::Attributes, args::Vector{LineString})
    @ Makie ~/.julia/dev/Makie/src/interfaces.jl:302
  [7] plot(P::Type{Any}, args::Vector{LineString}; axis::NamedTuple{(), Tuple{}}, figure::NamedTuple{(), Tuple{
}, kw_attributes::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Makie ~/.julia/dev/Makie/src/figureplotting.jl:48
  [8] plot
    @ ~/.julia/dev/Makie/src/figureplotting.jl:31 [inlined]
  [9] #plot#13
    @ ~/.julia/packages/MakieCore/msnkR/src/recipes.jl:34 [inlined]
 [10] plot(args::Vector{LineString})
    @ MakieCore ~/.julia/packages/MakieCore/msnkR/src/recipes.jl:33
 [11] top-level scope
    @ REPL[39]:1

[asinghvi17]

As I see it, there are two issues here (it's erroring on the first, but you'll run into the second):

1. The convert_arguments for LineString is not well implemented for this case. This is something we can fix relatively easily.
2. Primitives (and recipes) generally can't handle empty input. That might be a little more complicated :D

[rafaqz]

Oh no, so not so easy to fix...

The reason I want this is for MakieDraw.jl, where you really want to start with an empty plot and add things to it. Or sometimes want to delete everything and start from scratch. Currently I have to add a dummy geometry to everything so that they plot.

[asinghvi17]

Yeah, it's definitely a feature which should exist, we had something similar with some specially implemented buffer plot types but they weren't totally generic.

[jkrumbiegel]

Primitives (and recipes) generally can't handle empty input

That's not true, mostly these cases just hit type instabilities or reduce-over-empty-collection errors down the line. I fixed those for many cases where I needed them, for example you can easily set empty ticks on an Axis which gives you empty linesegments and text primitives.

[asinghvi17]

Yeah, we'd have to implement code to handle this kind of thing, without leaving observables undefined, in each recipe. For example - what would streamplot do with empty input? It can of course set the pipeline up, but not actually run it.

Backends would be an easier task in comparison, CairoMakie could do this pretty easily.

[jkrumbiegel]

In this specific case we just need to have a way to determine the type of the empty result vector correctly. Although Julia makes that tricky sometimes, for example Vector{LineString} has an abstract element type if I understand correctly, so you wouldn't be able to infer anything.

function convert_arguments(PB::PointBased, linestring::Union{Array{<:LineString}, MultiLineString})
    arr = copy(convert_arguments(PB, linestring[1])[1])
    for ls in 2:length(linestring)
        push!(arr, Point2f(NaN))
        append!(arr, convert_arguments(PB, linestring[ls])[1])
    end
    return (arr,)
end