Mean core velocities of four representative Fe I lines and a Si II line. The full timeline covers the years from the beginning of the 2000s until mid-2024. We note that there is a gap in the x-axis. Different symbols mark data from different spectrographs (mean error bars for each instrument are given in the top-left corner). The colours red, blue, orange, and green correspond to Fe I λλ6430,6358,6411, and 6322 lines respectively, and purple corresponds to Si II λ6347. A detailed look at each individual Fe I line during the dimming event is given in Appendix C.1.