Avoid area-size encoding for negative values
For quantitative reading with signed values, avoid area encoding on charts with sized marks to improve fidelity and mitigate larger-means-more misreadings for readers.
- purpose:refine
- basis:empirical
- data:quantitative
- quality:fidelity:use
- lever:encoding
- channel:area:avoid
advice
Area-size encoding removal
Do not use mark area to encode a signed measure. For example, avoid bubbles or sized circles when values can fall below zero, because larger areas still read as larger quantities.
reason
Why area breaks on negative values
Area works as a quantity metaphor only while bigger means more. Once the measure can go below zero, the size metaphor stops matching the meaning of the data.
Mechanism: Larger areas are firmly associated with larger positive amounts, so area cannot cleanly express both direction and magnitude for signed values.
Evidence: The paper states that area can convey quantity, but it cannot easily show negative values because larger sizes remain tied to larger positive quantities (Bertini et al., 2020).
context
Use when the measure can cross zero
- User Goal: Show a quantitative measure faithfully.
- Task: Read signed values, including negatives.
- Data: Numeric values that can be positive and negative.
- Chart Setting: The design is using or considering sized marks such as circles.
- Audience: Readers who need the sign as well as the magnitude.
- Success Criterion: The encoding does not make larger marks imply the wrong direction.
exceptions
Do not use this rule for non-negative magnitudes
Break it when: All encoded values are non-negative. Why: The larger-area-equals-larger-quantity metaphor still holds in that case.
costs
Costs of removing area-size encoding
Sacrifice: You give up a compact size-based display for that measure. Risk: If you keep area for signed data, negative values can look like larger amounts rather than values below zero. Mitigation: Move the signed field off area and reserve area for non-negative magnitude fields.
mistakes
Common sized-mark mistake
Mistake: Sizing marks by a value that can go negative. Why it fails: The display cannot keep the larger-means-more metaphor consistent across zero.
check
Check whether area is invalid for the measure
Failure Sign: The most negative values would require the largest marks or an impossible notion of negative size. Quick Check: Ask whether the encoding can cross zero without breaking the size metaphor. Stronger Test: If a larger mark ever stands for a smaller signed value, the area encoding has failed.
fix
Fix the signed-value encoding
- Remove area sizing from the signed field.
- Switch the signed field to a channel that can express both direction and magnitude.
- Keep area only for fields where larger always means more.