Makie slopeline length in screen space

makie_slopeline_screen_space.jl

using LinearAlgebra: normalize

f = Figure()
ax1 = Axis(f[1, 1])
slope1 = 2
data1 = [1 0; slope1 1] * randn(2, 500)
scatter!(ax1, data1, alpha = 0.1)
ax2 = Axis(f[1, 2])
slope2 = -0.5
data2 = [1 0; slope2 1] * randn(2, 500)
scatter!(ax2, data2, alpha = 0.1)

linkyaxes!(ax1, ax2)

function slopeline!(ax, pos, slope; length = 50, kwargs...)
    points = lift(ax.scene.viewport, ax.scene.camera.projectionview) do _, _
        pos_screen = Makie.project(ax.scene, pos)
        pos_delta = Makie.project(ax.scene, pos + Point(1, slope))
        d = normalize(pos_delta - pos_screen)
        offset = d * length / 2
        [pos_screen - offset, pos_screen + offset]
    end
    lines!(ax, points; space = :pixel, kwargs...)
end

slopeline!(ax1, Point2(0, 0), slope1, linewidth = 2)
slopeline!(ax2, Point2(0, 0), slope2, linewidth = 2)

f