This example uses D3 v5 to render an interactive multi-series line chart comparing city temperatures in both Fahrenheit and Celsius. A dropdown menu toggles the unit of measurement, triggering animated transitions of the lines and axes. The chart displays daily temperature data across three cities with a time-based x-axis. Hovering reveals a vertical guide with circles and labels, showing exact values for each city. The code uses `d3.line` with `d3.curveCardinal` for smoothed lines, `d3.csv` for data loading, `d3.scaleTime` for the date scale, and D3’s selection and transition APIs for interactivity, all rendered as SVG.