Scatter: scatterBuckets[layerId][col] lists the Chart.js dataset indices sharing that X.
Bar/line: barLineIndices[layerId][row][col] is the original Chart.js element index (gaps skipped).
Heatmap: heatmapIndices[layerId] maps "x\0y" to the flat Chart.js element index.
Per-layer lookups that translate a MAIDR navigation position into the original Chart.js element index. MAIDR extraction skips gap markers, so these maps re-derive the raw indices from the chart to keep highlights aligned.