Observed and model axial ratio, ζ, of the X-ray surface-brightness distributions of the clusters of our sample. Upper panel: Observed distribution of ζ (black) compared to the distributions of ζ of the α_turb = 0 (red) and α_turb ≥ 0 (blue) models. Lower panel: Observed (squares) ζ compared with the median values of ζ obtained for each cluster (as indicated in the vertical axis) for the α_turb = 0 (circles) and α_turb ≥ 0 (triangles) models. The error bars indicate the 16th and 84th percentiles measured in our models. The observed data points have very small 1σ uncertainties (δζ/ζ ∼ 10⁻³), which are not shown in the plot.