This scatterplot shows the relationship between engine displacement and horsepower for a set of cars, with each point sized and colored to represent data density. The visualization fetches live data from a PHP endpoint via `d3.json` and renders it using the vega-lite-api library, which builds a Vega-Lite specification. Point opacity is set to 0.1 to reveal overlapping clusters, while tooltips display the car name using the vega-tooltip handler. The chart is configured with custom axis styling and fills the entire viewport.