Comparison between the best-fit (to thick- and inner thin-disc stars) chemical tracks for different total SFH normalisations. The values of the integral of the SFHs are given in the legend in panel a). Panel a) shows the different fitted SFHs, panel b) shows the evolution of [Si/Fe] with stellar age and is the distribution used to fit the data. Panel c) shows the evolution of [Si/Fe] with metallicity, and panel d) shows the metallicity evolution with stellar age. The points are adapted from Haywood et al. (2013). Open circles are outer thin-disc stars, solid circles are thick-disc and inner thin-disc stars.