This contour plot visualizes the Old Faithful dataset, showing the density of eruptions against waiting times as a smooth gradient of green-filled polygons. The code uses d3-contour’s `d3.contourDensity` to compute density estimates from the scatter data, and d3-scale-chromatic’s `interpolateGreens` for the sequential color scale. SVG rendering via D3 v4 draws the contours with `d3.geoPath`. An interactive mouse move adjusts the bandwidth of the density estimation from zero to forty, updating the plot in real time.