Multiple city temperature trends are visualized as colored line paths over time, with a legend identifying each city and sorting them by final temperature value. The D3.js v5 code uses `nest()` to group data by city, `scaleTime` and `scaleLinear` for axes, and `line().curve(curveBasis)` for smooth paths. A color legend is built with a custom reusable component, while gridlines and axis labels support reading the temperature and time scales. The data comes from a CSV file of one week of city weather observations.