Maps of the skewness (odd rows) and kurtosis (even rows) of the observed longitudinal (first column) and latitudinal (second column) proper motions of Gaia DR2. The number of sources per bin is shown in the third column. Each block corresponds, from top to bottom, to the f = σ_ϖ/ϖ cutoff of 0.15, 0.20, 0.25, and 0.30. The Galactic centre and the bar are denoted by the black spot at d_b = R₀ and the solid line, respectively. The half-length and orientation angle of the bar are set to 4.5 kpc and 25°, respectively.