This stacked bar chart visualizes the proportion of smartphone app usage across six age groups, from 18-24 to 65+, with each bar segment representing a category such as Social Media, Games, or Productivity. Built with D3 v7, the visualization uses `d3.stack()` to compute the stacked series from the `phones.csv` data and renders them as SVG rectangles. A linear `d3.scaleLinear` maps the usage proportions (0–1) on the x-axis, while a band scale positions the age groups vertically. Color encoding via `d3.scaleOrdinal` distinguishes the app categories, and an interactive legend lists each type alongside its corresponding color.