This heatmap visualizes concurrent user sessions across days of the week and hours of the day, with cell color intensity encoding the session count. The SVG-based chart uses `d3.scaleQuantile` with a nine-color YlGnBu palette and includes a legend showing quantile thresholds. It reads TSV data via `d3.tsv`, renders labeled day and hour axes, and animates color transitions with `transition().duration(1000)` when switching between two datasets using the dataset picker buttons.