ErrorBarPoint.y is optional because a band may draw only bounds,
but most producers of an interval do compute an estimate and drop the
sample when they cannot — and a ForestPoint requires one. This
names that guarantee so a producer can state it once, at the point where
it is actually enforced, instead of every consumer asserting it (#1047).
An ErrorBarPoint whose estimate is known.
ErrorBarPoint.y is optional because a band may draw only bounds, but most producers of an interval do compute an estimate and drop the sample when they cannot — and a ForestPoint requires one. This names that guarantee so a producer can state it once, at the point where it is actually enforced, instead of every consumer asserting it (#1047).