Comparison of the histograms of the asymmetry coefficients. (a,b) c_B is computed directly with the magnetic data function of (a) the time t and (b) the spatial coordinate x (Eq. (2), x includes the ageing effect). (c,d) c_B′,x′ is computed after the ageing effect is removed from the spatial coordinate (x′, Eq. (27)) and the magnetic field (Eq. (28)). The expansion factor, f(t), is derived with (c) a linear fit, and (d) a quadratic fit of the observed velocity (Eq. (15)).