fill_between.jl

using Makie
N = 100
scene = mesh(
  FRect3D(Vec3f0(-0.5), Vec3f0(1)), color = :skyblue2,
)
rect = scene[end] # last plot is the rect
# there are a couple of ratate! functions, that accept e.g. a vector etc
rotate!(rect, Quaternionf0(0, 0.4, 0, 1))
scene

function create_poly(x, y1, y2, where::Nothing)
  x, y1, y2
end
function create_poly(x, y1, y2, where::Function)
  create_poly(x, y1, y2, where.(x, y1, y2))
  view(x, bools), view(y1, bools), view(y2, bools)
end
function create_poly(x, y1, y2, bools::AbstractVector{<: Union{Integer, Bool}})
  polygon = Vector{Point2f0}[]
  b_state = iterate(bools)
  poly1 = Point2f0[]
  poly2 = Point2f0[]
  for (i, b) in enumerate(bools)
    if b
      push!(poly1, Point2f0(x[i], y1[i]))
      pushfirst!(poly2, Point2f0(x[i], y2[i]))
    else
      isempty(poly1) || push!(polygon, unique!(vcat(poly1, poly2)))
      poly1 = Point2f0[]
      poly2 = Point2f0[]
    end
  end
  isempty(poly1) || push!(polygon, unique!(vcat(poly1, poly2)))
  polygon
end
function fill_between!(x, y1, y2; where = nothing, scene = AbstractPlotting.current_scene(), kw_args...)
  polygon = create_poly(x, y1, y2, where)
  for poly in polygon
    poly!(scene, poly; kw_args...)
  end
  scene
end

x = -5:0.01:5
y1 = -5 .* x .* x .+ x .+ 10
y2 = 5 .* x .* x .+ x
scene = lines(x, y1)
lines!(x, y2)

fill_between!(x, y1, y2, where = y2 .> y1, color = :yellow)
fill_between!(x, y1, y2, where = y2 .<= y1, color = :red)
scene