A small multiples line chart shows population growth from 1950 to 2015 for China, India, and the U.S. It uses D3 v3 with SVG, `d3.svg.line`, `d3.time.scale`, and `d3.legend.color`, rendering colored strokes per country. The y-axis uses a custom SI format (B for billions), and data loads from a CSV via `d3.csv`.