[BUG] can't render stacked unit chart #102
Description
The Stacked unit chart Observable Plot relies on implicit stacking of example converts to SveltePlot as:
<Plot
marginLeft={60}
x={{ label: 'Frequency →' }}
y={{ label: null }}
color={{ legend: true }}>
<BarX
data={penguins}
y="species"
x={1}
inset={0.5}
fill="body_mass_g"
sort="body_mass_g" />
<RuleX data={[0]} />
</Plot>However, this renders with an error about "duplicate key in BarX.svelte".
(This is one of the issues surfaced by #72)