This example draws a single horizontal line across an SVG canvas to illustrate ordinal positioning within a chart. The line, rendered with an arrow marker on each end, spans the width of the viewport with a 24-pixel margin. Using d3.v4, the code selects the SVG element and appends the line, then relies on the browser’s `getBoundingClientRect()` method to position two HTML labels, “Category A” and “Category B,” just below the line’s endpoints. The labels are placed via absolute CSS positioning within a container div, with their left offsets calculated from the line’s start and end coordinates minus the label’s own width.