Grid of 375 evolutionary sequences of single stars. Each evolutionary sequence of our grid is represented by one dot in this diagram. Sequences inside the blue shaded region follow chemically-homogeneous evolutionary paths evolving bluewards in the HR diagram and having a surface helium abundance of Y_S ≃ 0.98 at the TAMS. Sequences inside the red region follow normal evolution, keeping Y_S close to the initial value of ≃0.24. Sequences inside the yellow region deviate from normal evolution: either they start their lives evolving chemically homogeneously and then switch to normal evolution, or they start normal evolution and mass-loss uncovers their helium-rich layers (cf. Sect. 7). Diamonds mark the sequences that have not reached the TAMS (i.e. the calculation was stopped between 0.82 <Y_C< 0.98), and the yellow-dashed pattern indicates that the separation line between the red and yellow regions is uncertain at the highest masses.