This heatmap shows daily mean concentrations of four air pollutants (NO2, O3, SO2, and CO) over time, with color intensity indicating pollution levels. The visualization uses D3.js v6 to read a CSV file of EPA air quality data, parse dates, and render an SVG with a color scale based on the `d3.scaleSequential` with the YlGnBu interpolator. Each cell represents a pollutant-date combination, and hovering reveals a tooltip with the exact date and value. The chart employs `d3.axisTop` and `d3.axisLeft` for labeled axes.