This heatmap visualizes monthly COVID-19 cases across Scandinavian countries for 2020, using a blue sequential color scale to encode case counts per country-month combination. The SVG chart, built with D3 v7, reads CSV data from a GitHub gist and renders axes, labels, and tooltips. It uses `d3.scaleBand` for positioning, `d3.scaleSequential` with `d3.interpolateBlues` for coloring, and the d3-legend plugin for the color legend.