This heatmap visualizes concurrent user sessions by day of the week and hour of the day, using an SVG grid of 24 columns (hours) and 7 rows (days), with color intensity derived from a quantile scale of session counts. The example loads tab-separated data via `d3.tsv` and renders animated transitions between two datasets, selected by buttons, with each cell’s fill updating over 1000 milliseconds. It builds the chart with D3 v3, including `d3.scale.quantile`, SVG `rect` elements with rounded corners, axis labels for weekdays and times, and a legend showing value thresholds. Data originates from a development environment’s session logs, as noted in the README.