Line chart encodes values as position of a polyline through ordered time. William Playfair, 1786 — later refined for meteorology and markets.
In reports, ads and assignments it fits trends, seasonality and a few series over time. Skip it when unordered categories, or more than about four overlapping series.
Rules
- Rules 1 — Use position of a polyline through ordered time as one channel, not two.
- Rules 2 — Direct labels beat a distant legend.
- Rules 3 — Do not hide zero. If you break the axis, say so.
- Rules 4 — Colour encodes category or magnitude, not both.
- Rules 5 — Put the claim in the title, not just “Line chart”.
- Rules 6 — Source and period, even in small type.
Palette
- Palette 1 — Categories: distinct hues. Sequences: one hue, changing value.
- Palette 2 — Diverging: a neutral centre, emphasis at both ends.
- Palette 3 — One brand colour plus greys beats a rainbow.
- Palette 4 — Check contrast on a projector and in greyscale print.
Type
- Type 1 — Tabular figures for numbers. Do not pour body copy into the chart.
- Type 2 — Titles as sentences; axes as short nouns.
- Type 3 — For CJK, drop weight one step and leave tracking.
Line chart: honour position of a polyline through ordered time, and switch when unordered categories, or more than about four overlapping series.