# Area Chart — Situational Evidence

Use when accumulated magnitude, volume, or part-to-whole change across an ordered axis is the main question. Prefer a line chart when only the trajectory matters.

## Required

- Use a meaningful baseline, normally zero, because area encodes magnitude from that baseline.
- Use the selected colourmap and keep area fills subordinate to boundary lines and labels.
- For stacked areas, preserve a stable series order and provide a legend outside the plot.
- State whether values are absolute or normalized to 100%.

## Allowed choices

- Use 20–45% fill opacity when a boundary line carries the precise trajectory.
- Use a single area for cumulative volume or one dominant series.
- Use stacked area for a small number of parts when total and composition both matter.
- Use small multiples instead of stacking when individual trajectories need accurate comparison.

## Forbidden

- Do not use area fill as decoration beneath an ordinary line chart.
- Do not stack positive and negative values in a conventional area stack.
- Do not use more layers than can be traced reliably; switch to small multiples or lines.
- Do not reorder layers across time.