An interactive stacked and grouped barchart displays multiple data series generated by the `bumps()` function in `testdata.js`. The chart’s primary view is a stacked barchart, with a radio-button form allowing the user to toggle the layout to a grouped barchart, triggering an animated transition. The visualization uses D3 v6 scales, including `scaleBand` and `scaleOrdinal`, and a `d3.stack` operation to compute the stacked series. An SVG tooltip, populated with a legend of series values, appears on mouseover events.