A horizontal tree diagram visualizes hierarchical data with color-coded nodes representing question-answering states. Users toggle node expansion by clicking, with smooth transitions animating node positions and link paths. The visualization uses D3 v5's `tree` layout, `hierarchy` function, and `linkHorizontal` generator to compute the layout, while SVG elements and `transition` handle the rendering and animation. Interactive zooming is supported through the `zoom` API. The dataset comes from a static JavaScript object defining a tree structure with visit, attempt, and correctness classifications.