Two lines represent two different functions in this D3.js v5 chart, showing one week of temperature data across cities worldwide. The visualization uses `d3.scaleTime` and `d3.scaleLinear` for axes, `d3.line` with `curveBasis` for smooth paths, and `d3.nest` to group by city. Data comes from Data Canvas - Sense Your City, rendered as SVG with a color legend and axis labels.