Velocity shift, Δv, measured on ten random mock sightline pairs with both the pixel-by-pixel (blue square scatter points, see Sect. 5) and model-based methods (red diamond scatter points, see Sect. 6.4) where a different baseline is assumed between the two epochs. The horizontal blue dotted and dashed red lines report the average Δv measured with the pixel-by-pixel and the model-based methods, respectively. From left to right, the mock datasets are built assuming no shift (Δv_exp = 0 m s⁻¹), Δt = 0.875 yr (the same time elapsed between the two observed spectra, Δv_exp=−0.38 cm s⁻¹), Δt = 10³ yr (Δv_exp=−4.3 m s⁻¹), and Δt = 10⁴ yr (Δv_exp=−42.9 m s⁻¹). The dashed grey lines report the assumed shift.