A Marimekko chart (also called a Mosaic Plot) is a two-variable 100% chart: both axes run from 0% to 100%, and every segment in the chart simultaneously encodes its value on both axes. The x-axis encodes each group's share of the total (column width). The y-axis encodes the sub-category composition within each group (segment height). The result is a rectangle partitioned into cells where cell area directly encodes each category-group combination's share of the grand total. A cell whose column is 35% wide and whose segment is 42% tall occupies exactly 14.7% of the total chart area — which equals 14.7% of the total data. This area-as-proportion property is the chart's defining characteristic and its primary cognitive demand.
Every cell in a Marimekko simultaneously encodes four pieces of information: group identity (x-position), group share of total (column width), sub-category identity (fill color with redundant text label where space permits), and sub-category share within the group (segment height). The cell area synthesizes these into a fifth: the share of the grand total. No other single chart type encodes all five simultaneously without adding a fifth visual dimension (size, position, or annotation). This compression is Marimekko's strength — and the source of its notorious legibility ceiling at high cell counts.
A 100% stacked bar chart shows the y-axis composition correctly but assigns equal column width to all groups — the viewer cannot see that Sub-Saharan Africa receives 35% of total aid while Other Regions receives 7%. The column-width variable disappears. A treemap encodes area as proportion-of-total and handles arbitrary nesting, but loses the aligned y-axis that makes within-column composition comparison possible — the viewer cannot scan horizontally to compare "how much of each region's aid goes to Health." Marimekko is the only chart that preserves both comparisons simultaneously.
Marimekko charts require a specific reading contract: the viewer must first understand that column width ≠ data value, but rather encodes a separate variable. This is counterintuitive — in a standard bar chart, width is decoration. Here it is data. The chart therefore requires more viewer calibration than a simple bar chart. The FT Visual Vocabulary lists Marimekko under Part-to-whole with a note: "suitable for a general overview rather than precise comparison." For precise segment comparison, a small-multiple of standard 100% bar charts is preferable. For revealing the relationship between the two variables simultaneously, Marimekko has no close substitute.
The FT Visual Vocabulary classifies Marimekko under Part-to-whole: "Show how a single entity can be broken down into its component elements." It simultaneously touches Comparison: "Compare multiple items." Abela's framework routes here when the question is "how do the sub-category shares compare across groups, and how do the groups compare in total size?" — a bivariate composition question that requires both axes to carry distinct information. Tufte's data-ink principle is applied: the cell area itself is data (share of grand total), so every pixel of the filled rectangle carries information. The 2px column gap is the sole non-data element — it costs two pixels per boundary to purchase legible column separation, which passes the data-ink test.
Cell percentage labels are shown only when the cell is large enough to hold
them without overlap — minimum 35px wide and 18px tall. Smaller cells carry
no label; the tooltip delivers the value on demand. This is a deliberate
progressive disclosure strategy: forcing labels into small cells
would require font sizes below 8px (illegible) or rotated text (disorienting in a
chart that already demands careful reading). The tooltip for any cell shows four
values: region, sector, within-region share, and
share of grand total = xShare × segValue ÷ 100.
The grand-total area calculation is surfaced explicitly in the tooltip because it
is the chart's least-intuitive output — the number the viewer most needs and
is least likely to compute mentally.