A 3×3 grid of rectangles displays a static gradient of grayscale tones. Users select a column value from a menu, triggering a transition that outlines rectangles in the selected column and those before it with a red stroke. The visualization uses D3 v7’s selection, data join, and transition API to animate the SVG elements. The fixed grid data is hardcoded, and the menu is built with D3’s `dispatch` and standard HTML select elements.