First and third columns: PS in two energy bands of NICER observations of MAXI J1820+070 covering the HSS-to-LHS transition. The 0.3 − 2 keV data are shown in blue while the 2 − 12 keV one is in red, both with the best-fitting model (solid line) consisting of a combination of Lorentzian functions (dotted lines). Second and fourth columns: Real and imaginary parts of the CS between the same two energy bands rotated by 45°. We plot Re cos(π/4)−Im sin(π/4) in blue and Re sin(π/4)+Im cos(π/4) in red, with the best-fitting model assuming the constant phase-lag model. In each panel, residuals with respect to the model, defined as Δχ = (data − model)/error, are also plotted. The dashed orange vertical line in each panel corresponds to the centroid frequency of the real/imaginary QPO in the model.