posthog/formatting-insight-axes

Formatting insight axes

PostHog renders TrendsQuery insights with a built-in axis formatter. Use it instead of contorting formula or aggregationAxisPostfix to fake units.

The anti-pattern

If you are reaching for any of these, stop and pick a format below first:

• formula: "A / 60" with aggregationAxisPostfix: " mins" — manual seconds -> minutes
• formula: "A / 1000" with aggregationAxisPostfix: " s" — manual ms -> seconds
• formula: "A * 100" with aggregationAxisPostfix: "%" — manual ratio -> percent
• aggregationAxisPostfix: "ms" / "s" / "min" / "hr" on raw values

These freeze the unit at one scale. The built-in formatter picks a friendly unit per value (1.5s, 2m 12s, 1h 4m) and keeps the underlying series numerically correct for further math, breakdowns, and alerts.

Available formats

Set trendsFilter.aggregationAxisFormat on the TrendsQuery:

| Value | Use when the series is... | Renders as | | ------------------- | ---------------------------------------- | --------------------------- | | numeric (default) | a plain count | 1,234 | | duration | seconds (any scale) | 45s, 2m 12s, 1h 4m | | duration_ms | milliseconds | 850ms, 1.5s, 1m 4s | | percentage | already 0-100 | 47.3% | | percentage_scaled | a ratio 0-1 | 47.3% | | currency | money in the project's base currency | $1,234.56 (or local code) | | short | large counts you want compacted | 1.2K, 3.4M |

Companion fields on trendsFilter:

• aggregationAxisPrefix — literal prefix (e.g. "$") when you need a symbol pinned to a specific currency or unit, regardless of project settings
• aggregationAxisPostfix — literal suffix; reserve for genuine units the format can't express (e.g. " req", " events"), never for "mins" / "s"
• decimalPlaces — cap decimals (1 or 2 is usually right for currency / ratios)

Currency — pick format or prefix carefully

aggregationAxisFormat: "currency" renders with the project's base currency (set in project settings, defaults to USD). Use it when the underlying values are in that same currency — e.g. revenue events that PostHog auto-converts to the project's base currency.

If the values are pinned to a specific currency regardless of project (e.g. $ai_total_cost_usd is always USD, even on a EUR-base project), use aggregationAxisPrefix: "$" + decimalPlaces: 2 so the symbol matches the data. Using format: "currency" here would render USD values with € on a EUR project.

When the series is in seconds

If the series is in seconds (latency, session length, time-to-first-event, processing time, page load, etc.), silently default to aggregationAxisFormat: "duration". Do not stop to ask — the formatter is non-destructive (the underlying values stay in seconds either way, only the labels change), so picking it is always at least as good as raw seconds.

Only confirm with the user when they have explicitly named a fixed unit they want pinned ("show this in minutes", "graph the average in hours"):

"I can pin the y-axis to minutes by dividing the series by 60, or use PostHog's duration formatter which auto-picks seconds / minutes / hours per value — 90s renders as 1m 30s and 5400s as 1h 30m. Which would you prefer?"

In one-shot MCP contexts where no user is in the loop, just pick duration and move on.

Examples

Latency — duration in milliseconds

{
  "kind": "TrendsQuery",
  "series": [
    {
      "kind": "EventsNode",
      "event": "$pageview",
      "math": "p95",
      "math_property": "$performance_page_loaded"
    }
  ],
  "trendsFilter": {
    "aggregationAxisFormat": "duration_ms"
  }
}

Average session length — duration in seconds

{
  "kind": "TrendsQuery",
  "series": [
    {
      "kind": "EventsNode",
      "event": "$pageleave",
      "math": "avg",
      "math_property": "$session_duration"
    }
  ],
  "trendsFilter": {
    "aggregationAxisFormat": "duration"
  }
}

Revenue — currency in the project's base currency

{
  "trendsFilter": {
    "aggregationAxisFormat": "currency",
    "decimalPlaces": 2
  }
}

Fixed-currency value (e.g. LLM cost in USD) — pin the symbol

{
  "trendsFilter": {
    "aggregationAxisPrefix": "$",
    "decimalPlaces": 2
  }
}

Conversion rate — percentage from a 0-1 formula

{
  "kind": "TrendsQuery",
  "series": [
    {
      "kind": "EventsNode",
      "event": "checkout_completed",
      "math": "dau"
    },
    {
      "kind": "EventsNode",
      "event": "checkout_started",
      "math": "dau"
    }
  ],
  "trendsFilter": {
    "formula": "A / B",
    "aggregationAxisFormat": "percentage_scaled",
    "decimalPlaces": 1
  }
}

Updating an existing insight

If you are updating an insight and notice it already uses the formula/postfix anti-pattern, fix it in the same posthog:insight-update call — drop the divide-by-N, drop the aggregationAxisPostfix, and set the matching aggregationAxisFormat. The series values stay the same, only the labels change. Do not go scanning unrelated insights for this pattern — fix only the ones you are already touching.