Difference between the median transverse velocity in Galactic longitude as a function of longitude and distance for symmetric Galactic longitudes ℓ > 0 and ℓ < 0. We use bins of Δdcos(b) = 0.2 kpc and Δℓ = 10°. We plot a grey cross in bins where V_ℓ( + )−V_ℓ( −) is statistically consistent with 0 with a 95% confidence, i.e. where the observations are compatible with an axisymmetric galaxy. The grey region shows bins with insufficient data (we require at least five stars in each ℓ > 0 and ℓ< 0 bin). We have fixed the colour scale to 20 km s^-1.