A fact of a feed's schema in a design (spec/14 §7.4, D-136).
Visualize.Chart.fact/3 builds one: the attribute key of field in the schema bound
to slot source — a field's label, its unit, the order of its states. The term is
allowed wherever a value can appear, and in a text as the hole
#{fact(:source, :field, :key)}; Visualize.Chart.apply/2 replaces it by the
attribute, as it replaces a variable by its value. Reading a fact is a requirement: a
schema that does not give the attribute fails application, naming the slot, the field
and the attribute.
Examples
iex> Visualize.Chart.fact(:primary, :latency, :unit)
%Visualize.Chart.Fact{source: :primary, field: :latency, key: :unit}