A bar chart shows the count of days in Texas falling into four AQI categories for sulfur dioxide, from Good to Unhealthy. The chart is built with D3 v5, using `d3.scaleBand` and `d3.scaleLinear` for axes, and SVG elements for rendering. Hovering over a bar dims other values, highlights the selected bar, draws a dashed threshold line via a dynamically appended `line`, and displays the numeric difference between each category and the hovered one.