The visualization displays weekly influenza-like illness (ILI) totals across US regions from 2015 to 2019, with each year rendered as a colored line and point size encoding the same year. It uses the vega-lite-api to build a layered mark specification, with d3’s `csv` function fetching the raw CDC FluView data from a Gist. The chart is rendered via vega-lite and vega, with tooltips provided by vega-tooltip and visual styling customized through a shared config object.