Left panel: PS of NICER observation 1200120265 of MAXI J1820+070 in two energy bands. 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 5 Lorentzian functions (dotted lines). Right panel: 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 constant phase lags. In both panels, residuals with respect to the model, defined as Δχ = (data − model)/error, are also plotted. The dashed orange vertical line at 6.03 Hz in each panel corresponds to the centroid frequency of the QPO in the model.