Variation of R−ϕ of the phase spiral in the snapshot at t= 1.47 Gyr. Each panel shows z/h_z−v_z/σ_z map for a set of particles grouped by R and ϕ. The maps are colour-coded by the density contrast, Δ ρ. Panels exhibiting one-arm phase spirals are painted in red tone, while those with two-arm phase spirals are painted in blue tone. Panels where the phase spiral shapes cannot be classified as either one-arm or two-arm are painted in grey tones. The surface density contour of Fig. 9 is overlaid. While one-arm phase spirals are widely observed, two-arm phase spirals are only found in limited areas.