Illustration for the choice of optimal λ. The plot shows all residual functions R_m, n (grey lines, see Eq. (16)) and highlights the R_p0, q0 in red and Q (Eq. (17)) in blue. The fitted piecewise linear function Q_fit is shown in orange (see Eq. (18)). Vertical lines indicate the turning points of the fitted piecewise linear function Q_fit (orange dashed lines) and the finally adopted λ value at the intersection point of Q and Q_fit (blue dashed line).