The figure sync learns which row the pointer is in and that row's local y, so the shared x goes to every row and the horizontal line only to the hovered one -- a crosshair that finally marks a row where its y means something. The stroke gains Style::crosshair_width/_dash_on/_dash_off (logical; 0.75 / 4 / 4), a per-Crosshair override, and a fragment-shader dash on v_along (off == 0 is solid), anchored to the axes edge each line is staged from so the phase does not crawl.