3D Contour with 2D contour slices

using LinearAlgebra

using AbstractPlotting

function test(x, y, z)
    xy = [x, y, z]
    ((xy') * Matrix(I, 3, 3) * xy) / 20
end
x = range(-2pi, stop = 2pi, length = 100)
scene = Scene()
# c[4] == fourth argument of the above plotting command
c = contour!(scene, x, x, x, test, levels = 6, alpha = 0.3, transparency = true)[end]
xm, ym, zm = minimum(scene_limits(scene))
contour!(scene, x, x, map(v-> v[1, :, :], c[4]), transformation = (:xy, zm), linewidth = 2)
heatmap!(scene, x, x, map(v-> v[:, 1, :], c[4]), transformation = (:xz, ym))
contour!(scene, x, x, map(v-> v[:, :, 1], c[4]), fillrange = true, transformation = (:yz, xm))
# reorder plots for transparency
scene.plots[:] = scene.plots[[1, 3, 4, 5, 2]]
scene