Pure rotation disc model fit to PN velocities in M31 along the line of sight. The plot shows the fit for bin 8. The filled circles are the individual PN velocities v(x,y) expressed as function of the position angle on the sky (φ = 0 in Eq. (1) corresponds to PA = 35°; taken from the HyperLeda database). The red continuous curve represents the best-fitting rotation model, the red dashed lines are the model errors, as computed from the Monte Carlo simulations. Continuous black lines indicate the ± σ level, with the model errors indicated by black dashed lines. Black symbols refer to the analysed PN sample, blue symbols locate the discarded PNe: halo PNe, as defined by Sanders et al. (2012, filled blue circles), and satellite PNe, as identified by Merrett et al. (2006, open blue diamonds).