Three linked D3.js v7 views explore one year of synthetic air-quality data from eight urban stations: a deviation heatmap shows how daily PM2.5 departs from each station’s baseline, a slope chart compares winter and summer means, and a scatter plot fits `leastSquares` trend lines to NO2 versus PM2.5 for normal and wildfire-smoke days. Clicking any station highlights it across all views, while a `d3.brush` strip filters the date range. The SVG charts use `d3.scaleLinear`, `d3.axisBottom`, and `d3.group` for aggregation.