Change only starter/charts.py. Input: scenario/sales-clean/sales_daily.csv — clean long-format data (date, region, revenue; 135 rows, 45 days, 3 regions).

• load_sales(path) -> DataFrame with a parsed date column.
• summary(df) -> dict:
  • revenue_by_region — total per region, 2 decimals.
  • daily_revenue — total per ISO date, 2 decimals.
  • grand_total — 2 decimals.
• revenue_by_region(df, dest) — bar chart PNG at dest.
• daily_revenue(df, dest) — line chart PNG at dest.
• revenue_histogram(df, dest) — histogram PNG at dest.
• main() — writes out/revenue_by_region.png, out/daily_revenue.png, out/revenue_histogram.png, and out/summary.json (UTF-8, indent 2, trailing newline); prints a one-line confirmation; returns 0.

Constraints: matplotlib Agg backend only (no display); charts must be generated from the data, never hardcoded. Verification checks that the PNG files exist and are valid, and that summary.json's numbers match the dataset — it does not judge how the charts look.

Done means python verify.py starter exits 0 and python verify.py starter --expect-failure exits nonzero.