@jheinen, Plots.jl with gr() backend seems to exhibit several bugs on this example.
Bug on aspect_ratio=:equal, which is not applied to the z-axes:
surface(x, y, z', ratio=:equal)
Bug on the color scale limits if we set zlims to fix the aspect_ratio bug above:
surface(x, y, z', ratio=:equal, zlims=(-2,2))