The MAIDR data object, ready to be passed to <Maidr data={...}>.
Figure-unique layer id → original Chart.js dataset indices backing that
layer, in MAIDR row order. For axis-stacked panels each subplot only sees
a partition of chart.data.datasets, so MAIDR row indices no longer equal
Chart.js dataset indices — this map restores that correspondence.
Result of extracting a Chart.js chart, pairing the MAIDR schema with the bookkeeping the plugin needs to route navigation back into the chart.