Nitrogen abundance versus projected rotational velocity for 27 O-type stars with gravities log g_c ≤ 3.75. It constitutes all of the VFTS sources fulfilling these criteria, save for three O-dwarfs studied by Sabín-Sanjulián et al. (2014) that would all end up in Box 3 (Símon-Díaz, priv. comm.), binaries and poor quality fits (Sect. 3.4). Identical selection criteria have been applied to the population synthesis calculation projected on the background. The applied initial spin distribution in this calculation is assumed to be that derived for the presumed-single VFTS stars (Ramírez-Agudelo et al. 2013). Out of the 721 × 10³ simulated stars, 21 × 10³ pass the selection criteria (T_eff ≥ 29 kK and log g_c ≤ 3.75). These are normalised to 27 stars, matching the number of observed stars in the plot. The boxes, separated by the solid black lines, are defined in Sect. 5.2.