This example visualizes daily average air pollution levels for multiple cities across six pollutant types (CO, O3, NO2, PM10, PM2.5, and SO2) using horizon charts. Data is loaded from a CSV file, aggregated from hourly measurements, and each chart displays a time series for a city-pollutant pair with a color scale specific to the pollutant. The code uses D3 v4, the d3-horizon-chart plugin, and Canvas rendering via the horizon chart’s internal draw method. Charts are appended to the body with titles and computed average values.