Rule of Shared Scales
Comparable charts use comparable scales.
Requirement
Charts comparing absolute magnitudes of the same measure must use the same units, axis domains, scale transformations, and plotting dimensions. Time comparisons must use matching windows or explicitly labeled corresponding periods. Changes in layout must preserve the interpretation of the scale.
Why
Visual differences should reflect differences in data. Independently rescaling each panel can make very different magnitudes appear equivalent.
Exception
Separate scales may support comparison of patterns within each series. Label those scales clearly and identify the comparison as one of shape or relative change. Logarithmic and indexed views must identify their transform and reference value.
Examples
The pair isolates this rule. A good example is not approval of the entire interface.
Good for this task
Two carrier charts use the same 0–400 parcel scale, so 240 and 360 remain visibly different.
Parcels today
Bad for this task
Each chart rescales independently, making 240 and 360 appear almost identical.
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.