# Line Chart — Situational Evidence

Use for values measured across ordered time or another continuous progression. Do not use to connect unrelated categories or imply observations between unordered points.

## Required

- Put the ordered variable on the x-axis and the measured variable, with units, on the y-axis.
- Preserve missing intervals as gaps unless interpolation is analytically justified and disclosed.
- Use one contrast-safe stop from the selected colourmap per series. Keep the mapping stable across related charts.
- Use a 1.5–2.5px data-line stroke at ordinary screen sizes. Choose within that range from output size and density; use the same weight for peer series.
- Direct-label a small number of series when space permits; otherwise use a legend outside the plot.

## Allowed choices

- Use point markers when individual observations matter, sampling is sparse or irregular, or hover targets need reinforcement. Omit them for dense continuous series.
- Use stepped interpolation only for state changes or quantities that remain constant between events.
- Use a logarithmic y-axis for multiplicative change when values and reader expectations support it; label it explicitly.
- Emphasize one selected series with a stronger stop or up to 0.5px additional weight when selection has real meaning.

## Forbidden

- Do not smooth raw observations unless smoothing answers the question and the method is disclosed.
- Do not bridge missing data with an ordinary solid line.
- Do not force zero when doing so hides meaningful variation; do not truncate the axis to exaggerate negligible change.
- Do not use an area fill merely to decorate the line.
- Do not use one color per point unless color encodes another real variable.