An area draw layer: fills between a value column and a baseline, with a
graded (gradient) shade — opaque at the line, transparent at the baseline —
and an outline stroke on top. Reads column into a ChartSeries
(columnar, gaps as NaN), registers into the enclosing Layers (scaling
against its axis), and renders nothing to the DOM — the row draws it. The
fill + outline break at gaps rather than spanning them.
The fill rests on baseline (see AreaChartProps.baseline): 0
by default (positive up, negative down), any other number for a different
reference level, or 'floor' for the bottom of the plot. The esnet
two-colour traffic look composes two layers, each with its own as role:
<Layers>
<AreaChart series={s} column="in" as="in" />
<AreaChart series={s} column="out" as="out" />
</Layers>