A line graph encodes quantitative values as position along the y-axis — the most accurate perceptual channel in Cleveland and McGill's hierarchy — and connects successive points with a line that implies continuity. The line itself is not a data encoding; it is a visual metaphor for the path a value took between measurements. An upward slope means increase; a downward slope means decrease; diverging lines mean a growing gap. These readings are pre-attentive — the direction is understood before any axis is read. That metaphor only works when the x-axis is genuinely continuous (time, distance, sequence), never when it is categorical.
The data has a continuous time axis (annual, 2016–2024) and three series whose divergence is the message. A line graph is uniquely suited to divergence stories: the visual distance between lines at any x-position directly encodes the gap. In 2016 the three lines start close together; by 2024 the space between them is dramatically wider. No other chart form renders that expanding space as immediately. The y-axis is anchored at 0 — not the data minimum — because the message is about absolute levels and how far the bottom sits from the top, not about slope alone.
A grouped bar chart of this data requires 9 year-groups × 3 bars = 27 bars. Each year-group reads as a comparison at a moment rather than a trajectory over time. The viewer must mentally connect the high-income bars across all nine groups to perceive the trend — work the line chart does automatically. More critically, the growing gap between series cannot be seen in grouped bars without computing differences. The "gap" annotation that appears via the toggle on slide 1 would be the only way to surface it — and in a bar chart, that annotation has no geometric backing.
FT Visual Vocabulary: Change over time — Trend. Abela quadrant: Comparison (items over time, many periods). Tufte: the line graph has among the best data-ink ratios of any chart form — every pixel of line is a data encoding. Gridlines are kept light and sparse; they are references, not decoration.
The one decision worth knowing: lines animate in order from highest final value to lowest — high-income first, low-income last. This is not alphabetical. It creates a deliberate reveal: the viewer sees the leading line establish a ceiling, then watches the gap widen as subsequent lines draw in below. The ordering is an editorial argument that the distance matters, not just the levels.