(upper panels) Elliptic flow relative to the spectator plane, $v_2\left\{\Psi_{\rm SP}\right\}$, and to the participant plane, $v_2\left\{2,|\Delta\eta|>1\right\}$ and $v_2\left\{4\right\}$, as a function of centrality in Pb-Pb (left) and Xe-Xe (right) collisions. (bottom panels) Ratios of the elliptic flow $v_2\left\{\Psi_{\rm SP}\right\}$ and $v_2\left\{2,|\Delta\eta|>1\right\}$ to $v_2\left\{4\right\}$. The dashed (solid) lines show the eccentricity ratios of $\varepsilon_{\rm RP}$ ($\varepsilon_2\left\{2\right\}$) to $\varepsilon_2\left\{4\right\}$ from the elliptic power model. The corresponding eccentricity ratios for TRENTo are shown as solid bands The error bars (open boxes) indicate statistical (systematic) uncertainties The bin-to-bin uncorrelated uncertainties and the correlated ones are combined for $v_2\left\{\Psi_{\rm SP}\right\}$ results. For the ratio $v_2\left\{\Psi_{\rm SP}\right\}/v_2\left\{4\right\}$, the ZDC scale uncertainties are shown separately as solid boxes centered at unity on the right side of the lower panels.