μ(v) magnification profile of C IV (in red) computed from the spectra of images B and C obtained in May 2003 and February 2018, and binned into 20 spectral elements, as in Fig. 2. The line profiles from image B (in blue) and C (in black) are continuum-subtracted, corrected for the macro-magnification ratio and arbitrarily rescaled for convenience. The error of μ(v) is shown in green.