Surface nitrogen enrichment factor, computed as the nitrogen mass fraction divided by the initial nitrogen mass fraction as a function of the fractional hydrogen-burning lifetime, t/t_MS, for models with initial rotational velocities between 0 and 600 km s⁻¹ and initial masses of 5, 15, and 25 M_⊙ as marked in the plot. Top, middle, and bottom panels: SMC, LMC, and MW compositions, respectively. The colour scale corresponds to the critical rotation fraction, v_rot/v_crit. For each metallicity, various nitrogen abundances, ϵ = 12 + log(N/H), are displayed as dotted black lines with the value given in the plots of the left column.