Evolution of gas- and mass-averaged stellar [Z/X] for the boxy SFH shown in Fig. 2, where [Z/X] is defined in Sect. 5.6. Simulations applying the IGIMF theory and fixed canonical IMF are shown as the orange and blue lines, respectively. The solid, dashed, and dotted lines represent the value calculated for gas phase, mass average, and luminosity average (approximation, see text), respectively. The horizontal dash-dotted lines mark the boundaries where the stellar yield table is changed (at [Z/X] ≈ [Z] = −0.26, −0.497, −0.933, see Sect. 4.4 for explanation), which causes a change in the speed of enrichment (but with a 10 Myr delay). The serrated shape is caused by our simplification that all the stars formed in a 10 Myr star formation epoch have an identical age.