This example shows the correlation between a country’s happiness score and its health expenditure per person using a scatter plot with a linear regression trendline. The visualization, built with D3.js v3 and SVG, plots each country as a green circle, with axes labeled in dollars and happiness scores, and overlays a blue best-fit line computed via a least-squares function that returns the slope, intercept, and R-squared value. Data is loaded from a CSV file hosted on GitHub, while the regression calculation uses native array methods like `map` and `reduce`.