A calendar heatmap visualizes migrant deaths in the Mediterranean across the year 2015, with daily values encoded as colored cells. The data comes from a CSV file, and each day’s color reflects its value using a five-color scale with defined breakpoints. The visualization groups the calendar by year, using D3.js version 3.5.6 to parse dates, nest data by year, and render SVG rectangles for each day, with week and weekday positioning derived from `d3.time.weekOfYear` and `getDay`.