Rule of Proportion
Visual magnitude reflects numerical magnitude.
Requirement
Ordinary bars representing amounts must start at zero. Filled area charts representing amounts must preserve a zero baseline. Bubble area must be proportional to its encoded value. Decorative 3D projections must not distort a two-dimensional quantitative comparison.
Why
The size of the visual mark must represent the quantity faithfully.
Exception
Range bars and timelines represent intervals; their endpoints must represent those intervals accurately. Unfilled line and scatter plots may use nonzero domains to reveal variation. Actual three-dimensional data may justify a three-dimensional view. Axis labels must make the chosen mapping clear.
Examples
The pair isolates this rule. A good example is not approval of the entire interface.
Good for this task
A bar representing 60 starts at zero and is twice the length of a bar representing 30.
Parcels today
Bad for this task
A truncated bar axis begins at 25, exaggerating the visual difference between 30 and 60.
Parcels today
These synthetic fixtures illustrate the stated comparison. Native checks can support review of declared scales, context, encodings, geometry, and legibility; they do not establish data truth or task relevance. Review the requirement and exceptions above.
Enforcement
automated is the strongest mechanism Viewrule expects for the complete rule. Partial checks can still provide supporting evidence without proving the whole requirement.