Flux ratios for the cores of the C IV and C III] emission lines. The filled purple diamonds and filled blue triangles represent delay-corrected C IV and C III] line-core flux ratios, respectively, and the horizontal dashed lines and strips describe the average values and their uncertainties (purple for C IV and blue for C III]). The open purple diamonds (C IV) and open blue triangles (C III]) represent single-epoch measurements from auxiliary spectra (see Appendix B). For comparison, we also show the long-term evolution of the delay-corrected r-band flux ratio (filled circles), along with a few single-epoch values before 2016 (open circles).