Particle paths for individual elements that start along a) y₀ = −x₀. b) y₀ = x₀. c) y₀ = x₀ looking down on the xy-plane, i.e. there is a line-of-sight effect along the z-direction and X-line is viewed from above. d) y₀ = x₀ looking down on the xz-plane, i.e. there is a line-of-sight effect along the y-direction and X-line is viewed end-on. Blue line denotes the X-line. Ray paths for | x₀ | < 0.252 are coloured red and are trapped by the X-line.