This heatmap visualizes the frequency of user sessions by day of week (rows) and hour of day (columns), with color intensity representing the recorded value. Built with D3 v3 and rendered as SVG, the chart includes an animated transition that smoothly swaps between two datasets (data.tsv and data2.tsv) when triggered. The color scale maps low to high values to a sequential palette, and hover tooltips reveal exact values. The example is inspired by Trulia Trends, uses concurrent user session data from a development environment, and is MIT licensed.