Fork of Template Enter, exit and update
This example demonstrates the D3.js enter, update, and exit pattern using animated SVG circles. Six circles initially appear, then one is removed after two seconds, and the first circle changes color after four seconds, with each state change triggering a two-second transition. The visualization uses `d3.select`, `d3.range`, `d3.scaleOrdinal`, and `transition()` to illustrate how data joins and lifecycle methods handle dynamic updates. The color scale maps object types to fill colors, though the initial and updated object types are not explicitly defined in the code.