Hertzsprung-Russell diagram of the random spectra (black dots) produced for the code performance tests (T_eff and log(g) are in K and cm/s², respectively). The isochrones in long-dashed, dotted, and dash-dotted lines correspond, respectively, to Z = 0.06 and 13 Gyr, Z = 10^-4 and 1 Gyr, and Z = 0.019 and 100 Myr. The higher density of random spectra between 7000 and 8000 K is a consequence of the temperature overlap between the two different reference grids, for hot and cool stars (see Sect. 3.1), from which the random spectra have been interpolated.