This interactive D3 v5 app lets a user mix and match colors, sizes, and shapes by clicking swatches in side panels, updating a central mark rendered as an SVG path. It uses `d3.symbol` with `symbolCircle`, `symbolSquare`, and `symbolTriangle` for the geometry, and `selection.merge` to handle dynamic re-renders of the panels and the mark on each state change. The interface is laid out with fixed SVG groups, and the mark’s fill and scale are driven by user selections from the color, size, and shape panels.