Spatio-kinematic diagrams for Alessi 3. The real data from Gaia EDR3 is plotted in the first row. The second row shows the result of our method. In the first column, we represent the star positions and their proper motion vectors. In the second column we show the total proper motion in function of the radius to the cluster’s centre. The third and fourth columns show a polar decomposition of the proper motion field. In the third column we show the component in the angular direction in function of the distance to the cluster centre, which indicates the rotational component of the proper motion: it is positive for anticlockwise rotations and negative for clockwise ones. In the fourth column, we show the radial component in function of the distance to the cluster centre, which can indicate if the cluster is contracting if it is negative, or expanding if it is positive.