pe-widget-arr-per-customer

# Recreate the ARR per Customer graph from the reporting pack

The portfolio's monthly reporting pack includes the graph shown in
`assets/target.png`: average revenue per active billing customer, blended across all active portfolio companies.

Recreate it as a standalone graph definition at `graphs/arr-per-customer.json`, using
the warehouse data and the graph definition contract and house conventions
documented in `contracts/README.md`. The graph takes no dashboard parameters.

Match the chart as rendered: its data, point labels, legend, axes, and theme
colors. The definition must keep producing the correct reporting window as new
monthly actuals land, without further edits.

Check your work with `node contracts/validate.mjs graphs/arr-per-customer.json`.