This example shows a single green circle that displays a custom HTML tooltip on hover, with the tooltip following the mouse position. The SVG circle and tooltip are created using D3 v4, with the tooltip built as an HTML div styled via inline CSS. Pan-and-zoom functionality is enabled through `d3.zoom`, which applies a transform to the SVG group. Mouse events use `d3.mouse` to position the tooltip relative to the cursor, using `mouseover`, `mousemove`, and `mouseout` handlers.