Centrality dependence of $v_4$ (left), $v_5$ (middle) and $v_6$ (right) in Pb--Pb collisions at $\sqrt{s_{_{\rm NN}}} = 2.76$ TeV. Contributions from linear and non-linear modes are presented with open and solid markers, respectively. The hydrodynamic calculations from ${\tt IP}$-${\tt Glasma+MUSIC+UrQMD}$  are shown for comparison.