Top left panel: PS of NICER observation 1200120268 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). Top 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. Bottom panels: Phase-lag spectrum (left) and intrinsic coherence function (right) with the derived model (solid line) obtained from the fit to the PS and CS. In all four panels, residuals with respect to the model, defined as Δχ = (data − model)/error, are also plotted. The dashed orange vertical line at 2.06 Hz in every panel corresponds to the centroid frequency of the imaginary QPO in the model.