A place in a chart, in the chart's own terms (its data, never pixels,
as each person has the chart at their own size): where a person's
pointer is, where an agent points (Issho's point tool). Its format is
part of Fude's public API, and only grows.
%{"datum" => %{"x" => "Europe", "color" => "Online"}} # a bar
%{"datum" => %{"x" => "2026-03-01"}} # a time on a line chart
%{"datum" => %{"x" => 120, "y" => 4.5}} # a point of a scatter
%{"datum" => %{"region" => "BRA"}} # a region of a map
%{"datum" => %{"x" => "Europe"}, "x" => 0.5, "y" => 0.2}"datum" names a mark by the values of its channels (x, y,
color, region): a category as its name, a time as ISO 8601 text, a
number as it is, a region by any of its codes or its name. A mark that
its values do not name all of (a time of a line chart with several
lines) is the place across the plot at that value. "x" and "y"
are where in that place, from 0 to 1, as Issho draws a pointer. In
the room, it is the "at" of a place of the chart's document:
%{"doc" => "report:7", "at" => %{"datum" => %{"x" => "Europe"}}}.