Hunter diagrams, showing stellar age (red to blue colour bar) as a function of rotation and nitrogen surface abundance. Evolutionary tracks are taken from Köhler et al. (2015) and depict the evolution of stars with an initial mass of 230 M_⊙ (top), 100 M_⊙ (centre), and 60 M_⊙ (bottom) with a range of initial rotation rates. We note that the theoretical rotation rates are scaled by π/4 to correct for the average projection angle, and that the tracks are cut off at T_eff < 29 000 K. The diamond markers indicate the observed positions of sources in the respective mass-regimes, where their colour maps to age via the same coding as the tracks. For a discussion on the error bars on the observed rotation rates of the WNh stars, see text.