Velocity-velocity diagram used to compare the non-parametric velocities of [O III], Hα, and Na ID gas for the 31 nuclei reported in Table 4. Each nucleus is identified with a distinct colour (see labels in Fig. 6). In the left column, we show the central velocities of [O III] vs. Hα (top) and of [O III] vs. Na ID (bottom); horizontal and vertical dot-dashed lines mark the zero-velocity positions, and the dashed line displays the 1:1 relation. In the central column, we show the v10 velocities, associated with the blue wings of [O III] and Hα (top), and of [O III] and Na ID (bottom); the dashed line indicates the 1:1 relation. In the right column, we show the v90 velocities, associated with the red wings of [O III] and Hα (top) and of [O III] and Na ID (bottom); the dashed line indicates the 1:1 relation.