Test-particle streams evolved in the axisymmetric Kuzmin-Kutuzov potential and shown in Figs. 9–11, depicted now assuming a spherical approximation to the potential in the space of frequencies ΔΩ_r−ΔΩ_φ (top panels) and angles Δθ_r−Δθ_φ (middle panels) and Δθ_r−Δθ_ϑ (bottom panels). The colours represent the energy gradient in the isochrone potential. The panels have been centred on the progenitor position. The insets indicate the fitted slopes for the approximate potential. The errors were found by bootstrapping the fit 200 times.