The pseudorapidity dependence of $\Delta v_1$ for charged particles (blue markers) and D mesons (orange markers) in mid-central Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV . The data points for charged particles are scaled by a factor $10^3$. Statistical and systematic uncertainties are represented by the error bars and the boxes around each data point, respectively.