This example demonstrates the enter, update, and exit pattern for circle elements in D3 v5, using two buttons to modify an array of data points. The "Remove" button pops the last item, turning the exiting circle red, while the "Modify" button replaces a random entry, updating the circle’s position via `scaleLinear` mappings. The code imports `select`, `scaleLinear`, and `max` from D3, and uses SVG rendering with a fixed 960×500 canvas.