// ============================================================================
// Query: Last N Month Cost Trend
// Description:
//   Returns total billed and effective cost by month for the last N months (default: 12).
//   Useful for visualizing both billed and effective cost trends over time.
// Author: FinOps Toolkit Team
// Parameters:
//   startDate: Start date for the reporting period (e.g., startofmonth(ago(365d)))
//   endDate: End date for the reporting period (e.g., startofmonth(now()))
// Output:
//   Each row represents a month with its total billed and effective cost.
// Usage:
//   Use this query to monitor monthly cost trends, compare billed vs. effective cost, and support budgeting/forecasting.
// Last Tested: 2025-05-17
// =========================================================================

let startDate = startofmonth(ago(365d)); // Default: 12 months
let endDate = startofmonth(now());
Costs()
| where ChargePeriodStart >= startDate and ChargePeriodStart < endDate
| extend x_ChargeMonth = startofmonth(ChargePeriodStart)
| summarize BilledCost = sum(BilledCost), EffectiveCost = sum(EffectiveCost) by x_ChargeMonth
| order by x_ChargeMonth asc