This example demonstrates how D3’s data join reconciles SVG circles with an array of values, updating existing elements and appending new ones via `.join('circle')`. The visualization pairs two fixed circles with a joined selection of four circles, whose radii derive from the data `[20,30,40,60]`. Using d3.select and d3.csv, the code reads `data.csv` and `data2.csv` but does not display their contents, instead focusing solely on the join mechanics. Rendered as SVG, the circles are styled with red, green, and blue strokes, highlighting how the join overrides attributes like position and fill.