This visualization displays the Old Faithful dataset as a binned 2D heatmap using Vega-Lite. It shows the relationship between eruption durations and waiting times, with each rectangular bin colored by the mean waiting time. The code builds a layered specification including a rect mark for the heatmap and a text mark for mean values. It uses `vegaEmbed` to render the chart, with `d3.tsv` loading the raw data from a Gist. The bin count is configurable via a `bins` parameter, and the color scheme is set to "greens".