Mass fraction of helium at the stellar surface (Y_S) as a function of that in the core (Y_C) for sequences of different initial masses and rotational velocities as indicated by the legend, in units of M_⊙−km s^-1. Dots mark every 10⁵ years of the evolution. The diagonal line (Y_C = Y_S) is marked by a lightblue (dashed) line. Stars that evolve chemically-homogeneously (CHE) lie close to the diagonal, while those that undergo normal evolution (NE) trace a horizontal line; stars with transitionary evolution (TE) lie between (see also Fig. 3). One non-rotating sequence (of type M) with M_ini = 300 M_⊙ from the LMC grid of Köhler et al. (2015) is shown for comparison.