This example demonstrates two SVG legends for mapping data to visual channels. The color legend uses a `scaleOrdinal` with domain categories and a range of colors, while the size legend uses `scaleSqrt` with five ticks to show proportional circle radii. Both legends are implemented as reusable functions, `colorLegend` and `sizeLegend`, which bind data to groups and update circles and text elements. The code uses D3 v5 selections, `enter`/`merge`/`exit` patterns, and SVG attributes like `r`, `fill`, and `transform` for layout.