Adjacent rectangles need borders or gaps to show hierarchy

What it is

Adjacent rectangles establish geometric contact, not whether they share a parent. A treemap needs hierarchical boundary cues such as borders, padding, parent tint, headings, or interactive highlighting to make common regions and nesting legible. The title does not make borders and gaps the only options or imply that heavier is always clearer. Cues must follow hierarchy while preserving useful data area.

Why it happens

When a parent outline and a child seam look alike, readers cannot tell which level owns the boundary. Differentiated spacing, contours, and parent labels can establish hierarchy, but padding reduces descendant area, strokes cover small blocks, and tint can compete with data colour. A rendered boundary does not create a programmatic parent-child relation; visual and accessible trees must derive from the same hierarchy.

Studying it

Compare no boundary, padding, parent contours, header bands, tint, and interactive focus while varying depth, density, colour, zoom, and print. Test same-parent judgment, ancestor recall, branch lookup, and share comparison; measure errors, time, and lost small blocks. Test high contrast, colour-vision variation, low vision, and screen-reader navigation separately. No single pixel width or same-parent error threshold transfers across devices.

Where it stops holding

Strong boundaries can dominate the graphic and damage area proportionality; weak ones disappear in low contrast or print. Colour-only parent grouping conflicts with leaf value or category colour and excludes readers who cannot distinguish it. Deep structures may need focus interaction or a hierarchical table rather than more border levels. Lines with existing meanings, such as administrative boundaries, cannot be casually reused as hierarchy cues.

Applying it

• Give parent regions, sibling seams, and selected paths clear, limited boundary roles, checking after layout whether padding has erased small-block readability.
• Place parent names beside their common regions. On focus, expose the ancestor path and same-parent extent while preserving keyboard state.
• Generate programmatic tree, breadcrumbs, and rectangles from the same data, exposing parent, depth, and child count rather than relying on colour or coordinates.
• Compare cues on same-parent, path, and share tasks across zoom, narrow screens, print, and high-contrast mode.

Related

• Same group: U2.11.1 Treemaps encode shares as areas of nested rectangles · U2.11.2 Deep-node areas get too small to read; cap the depth · U2.11.3 Layout algorithms fix aspect ratios; thin strips resist comparison · U2.11.5 Treemaps cannot show negatives or support cross-branch comparison
• Adjacent: U1.13.3 Common region can replace the legend for belonging · U1.14.2 Over-pruning damages readability
• Search terms: treemap padding · hierarchy boundary · common region · parent child