This layered scatterplot visualizes the relationship between fertility, life expectancy, and population for different clusters by country and year. The visualization uses two overlaid circle layers rendered with Vega-Lite and embedded via `vegaEmbed`. A base layer displays hollow gray circles of varying sizes to show a population size legend, while a main layer encodes cluster membership through the stroke color of additional hollow circles. The chart reads from an external `data.json` file loaded using `d3.json`.