OptionalwidthThe category's share of all observations, as a fraction of one -- the width its column is drawn at.
Carried on every cell of the column rather than once per column, the way
z is carried on every cell of a series: the grammar's unit is the
point, and a producer emitting a flat list has nowhere else to put it.
OptionalcountThe cell's own count, when the producer has the contingency table.
A mosaic is drawn from a two-way table, and the count is the number the table was built on. It is optional because a producer working from proportions alone genuinely does not have it, and inventing one by multiplying out a rounded share would put a number in the announcement that the data does not contain.
One cell of a mosaic (marimekko) plot.
A mosaic is a stacked bar chart in which the bar widths also encode data -- typically each category's share of all observations. A reader given only the segment heights has half the table: the conditional proportions without the group sizes they were computed from, so a category of six people and one of six hundred read identically.