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.
Parts
- Mark — The core mark of Line chart is position of a polyline through ordered time.
- Scale — The scale must show zero (or a declared break) and units.
- Axis — Axis titles name the variable, not a mood.
- Legend — A legend is weaker than a direct label. Write beside the mark when you can.
- Label — Numbers are optional. If comparison is the job, label the ends only.
- Grid — Keep the grid faint. Data should own the ink.
- Annotation — One annotation sentence. Date the source.
- Frame — A thin frame. Never a 3-D box.
Variants
- Time series — Variant 1 of Line chart. Same encoding, different layout.
- Trend line — Variant 2 of Line chart. Same encoding, different layout.
- Playfair line — Variant 3 of Line chart. Same encoding, different layout.
- Line chart 4 — Variant 4 of Line chart. Same encoding, different layout.
Labels
- Axis — On Line chart, make Mark readable.
- Legend — On Line chart, make Scale readable.
- Label — On Line chart, make Axis readable.
- Grid — On Line chart, make Legend readable.
- Annotation — On Line chart, make Label readable.
Line chart: honour position of a polyline through ordered time, and switch when unordered categories, or more than about four overlapping series.