The quantile, as a fraction from 0 to 1: 0 is the minimum, 0.5 the
median and 1 the maximum. TensorBoard's distribution dashboard draws
0, 0.0668, 0.1587, 0.3085, 0.5, 0.6915, 0.8413, 0.9332
and 1 -- the median and the points one, two and three standard
deviations either side of it for a normal distribution.
The value at that quantile, or null where the chart has a position but
no reading -- a gap, as LinePoint.y spells one.
One quantile of a PercentileBandPoint: the value below which a
levelshare of the distribution at that x falls.