Relationship between dV_a and for W Hya. Symbol size is proportional to feature size. Proper motions derived from features matched in 2001–2002 or at 3 epochs are shown by solid symbols. The other two-epoch proper motions, with high uncertainties as discussed in the text, are shown by hollow symbols. The linear error-weighted least-squares fit and dispersion for the 3-epoch and 2001–2002 data are plotted by the solid and dashed lines.