This example visualizes daily average air pollution levels for multiple cities across six pollutants (CO, O3, NO2, PM10, PM2.5, SO2) using stacked horizon charts. Each chart represents one city-pollutant pair, with a color scale unique to each pollutant type. The code loads D3 v4 via UNPKG and the d3-horizon-chart library, processes an air-pollution.csv file by computing 24-hour means, and uses `d3.csv` and `d3.select` to bind data and render the horizon charts.