Difference of the spectral indices between the case of variable and an MW-IMF for the model galaxies in the CR-IGIMF run (black dots). The top and bottom panel show the TiO2_SDSS and NaD indices, respectively. The green stars show the binned trend, with error bars marking the 5th and 95th percentiles of the distribution in each bin. The red curves correspond to the observed trends from La Barbera et al. (2013), corrected to an infinite aperture (see text for more details).