This bar chart displays the estimated 2015 total population of four transboundary river basins in Africa, sourced from an African Union dataset. The D3.js v5 code builds the visualization using `d3.scaleLinear` for the x-axis, `d3.scaleBand` for the y-axis, and `d3.axisBottom` with a custom tick formatter that converts large numbers to billions. Grid lines extend across the chart via negative tick sizes, while the y-axis labels are simplified. The horizontal bars are rendered as SVG `rect` elements with a fixed blue fill.