improve direction markers in waterfall recipe

[daschw]

Description

Firstly this avoids showing direction markers in the waterfall recipe when no bars are visible:

using AlgebraOfGraphics
using CairoMakie

df = (
    x=repeat(1:2; inner=4),
    y=[1, 0, 0, 1, -1, 2, 2, 0],
    group=repeat('a':'d'; outer=2),
)

draw(
    data(df) *
    visual(Waterfall; show_direction=true, show_final=true, direction_color=:black) *
    mapping(:x, :y; color=:group, dodge=:group),
)

before the change:

after the change:

Secondly, it allows to pass different types to marker_pos and marker_neg in the waterfall recipe:

draw(
    data(df) *
    visual(Waterfall; show_direction=true, show_final=true, marker_pos='↑', direction_color=:black) *
    mapping(:x, :y; color=:group, dodge=:group),
)

before the change:

ERROR: MethodError: Cannot `convert` an object of type Symbol to an object of type Char

Closest candidates are:
  convert(::Type{T}, ::Number) where T<:AbstractChar
   @ Base char.jl:184
  convert(::Type{T}, ::T) where T<:AbstractChar
   @ Base char.jl:187
  convert(::Type{T}, ::AbstractChar) where T<:AbstractChar
   @ Base char.jl:186
  ...

after the change:

Type of change

Delete options that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Added an entry in NEWS.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.

[jkrumbiegel]

thank you!