A scatterplot of city populations updates dynamically through buttons that add or remove data points. The visualization uses D3 v4 with an SVG rendering, mapping population to circle radius via a square-root scale and continent to color through an ordinal color scheme. The data join pattern selects all circles, binds the updated array, removes exiting elements, and styles entering and existing circles with the same attributes. Clicking Add generates a random Shanghai population, while Remove deletes the last city.