nika:chart

with:
  metrics: "${{ tasks.metrics.output }}"
invoke:
  tool: "nika:chart"
  args:
    data: "${{ with.metrics }}"   # rows · array of flat objects (strings + numbers) · or { path: <json file> }
    semantics: { period: timestamp, cost: usd, provider: category }
    chart:
      type: line                          # bar | line | area_band | scatter | heatmap (closed)
      x: period                           # field names · y_lo/y_hi = area_band bounds · y2 = actual overlay
      y: cost                             # color = series split (heatmap: the value field)
      title: "Cost over time"
    out: report/cost-over-time.svg        # .svg REQUIRED — SVG is the attestation surface
    compile_to: vega_lite                 # optional · also writes the .vl.json sibling