Index δ of the power law modifying the starburst attenuation curve versus the V band attenuation. The symbols are identical to those of Fig. 3. The black line corresponds to the fit of a function of the form a × exp(b×A_V) + c using an orthogonal distance regression method, with the gray area representing the 1-σ dispersion. The dashed green line represents the model of Chevallard et al. (2013) as described in Sect. 5.2. We see that there is a clear relation between the V-band attenuation and δ, with higher attenuation galaxies having a shallower attenuation curve. Such a variation is expected. It has been found in models and simulations alike.