Probability density distributions of stellar orbits P(λ_z, r) in the phase space of time-averaged radius r vs. circularity λ_z for the best-fitting model of a typical simulated spiral galaxy. The normalized probability P/P_max is indicated by the colour bar. We adopted the Voronoi-binning scheme in the λ_z-r plane to make roughly equal-weight orbit bundles. Their boundaries are represented by the blue lines.