This is a simple D3.js bar chart that visualizes a numeric dataset of 20 values, displaying each value as a rectangle scaled in height and colored in varying shades of blue. The chart uses an SVG element sized 500 by 100 pixels, with rectangles positioned horizontally and vertically scaled by a factor of four. Each bar is labeled with its corresponding number, rendered in white text. The example demonstrates basic D3 v4.5.0 APIs including `d3.select`, `svg.selectAll`, `.data()`, `.enter()`, and `.attr()`, with a static dataset defined as an array of integers.