This interactive contour plot visualizes the Old Faithful dataset, showing the density of eruption and waiting times through colored bands. The visualization displays contours rendered as filled SVG paths using `d3.geoPath`, with colors mapped from a sequential greens scale. Users adjust the density bandwidth by moving their mouse across the plot, which triggers a re-render of the contour data via `d3.contourDensity` and `d3.select` event handling. Axes and labels are drawn using `d3.axisBottom` and `d3.axisLeft`, and the density estimate uses a Gaussian kernel with a bandwidth that ranges from 0 to 40 pixels as the mouse moves horizontally.