Observable Plot allows variable fills for an area or line, as in this example.

This example would translate to SveltePlot as:

<Plot color={{ type: 'log', legend: true }}>
    <AreaY
        data={aapl}
        x="Date"
        y="Close"
        fill="Volume"
        z="null" />
    <RuleY data={[0]} />
</Plot>

However, this currently renders with a single color:

(This is one of the issues surfaced by #72)