The ObservablePlot Overlapping histogram example is:

Plot.plot({
  round: true,
  color: {legend: true},
  marks: [
    Plot.rectY(olympians, Plot.binX({y1: "count"}, {x: "weight", fill: "sex", mixBlendMode: "multiply"})),
    Plot.ruleY([0])
  ]
})

The rendered chart is unaffected by changing {y1: "count"} to {y2: "count"} (but changing it to {y: "count"} causes rectY to apply an implicit stackY transform).

The SveltePlot equivalent is:

<Plot y={{ grid: true }} color={{ legend: true }}>
    <RectY
        {...binX(
            { data: olympians, x: 'weight', fill: 'sex' },
            {
                y: 'count'
            }
        )} />
    <RuleY data={[0]} />
</Plot>

which renders like:

Changing to y1: 'count' makes it match the apeparance of the ObservablePlot plot:

I wanted to flag this discrepancy, but my first thought is that what SveltePlot does might be more reasonable.

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