Coefficients of a linear relation between the normalised line of sight velocity dispersion, σ_los/V_c, and the apparent elongation, q_2D, of the clusters. The shaded regions show the 68 and 95 per cent confidence contours of the posterior probability distribution marginalised over the intrinsic scatter in σ_los/V_c. A positive slope signifies a correlation between the normalised line of sight velocity dispersion and the apparent elongation, consistent with prolate cluster shapes both in 3D position and 3D velocity space.