Data wrangling

Outline (draft)

• The mismatch: APIs and CSVs hand you raw rows. Charts want nested groups, totals, sorted lists. Wrangling is the bridge.
• Recap of array methods students already know: map, filter, reduce, sort. The 80% case.
• d3.group: array → Map keyed by an accessor. Useful for one-line "give me a series per category".
• d3.rollup: same as group but with a reducer step. Compute totals, averages, counts per group in a single call.
• d3.cross and d3.cumsum: short mention for stacked or combinatorial layouts.
• Where to wrangle: once with useMemo, not on every render. Show what happens without it.
• Pitfall: mutating the original array. Always return new structures.