https://github.com/ampproject/amphtml/blob/07ecf6b/extensions/amp-analytics/0.1/vendors.js#L649 https://github.com/ampproject/amphtml/blob/master/spec/amp-var-substitutions.md

amp-analytics.googleanalytics.json

{
  "vars": {
    "account": "UA-00000000-0"
  },
  "requests": {
    "scrollEvent": "${host}/collect?${basePrefix}&t=event&ec=${eventCategory}&ea=${eventAction}&el=${eventLabel}&ev=${eventValue}&ni=${ni}${baseSuffix}",
    "scrollTiming": "${host}/collect?${basePrefix}&t=timing&utc=${timingCategory}&utv=${timingVar}&utt=${timingValue}&utl=${timingLabel}${baseSuffix}"
  },
  "triggers": {
    "trackPageview": {
      "on": "visible",
      "request": "pageview"
    },
    "scrollPercentage": {
      "on": "scroll",
      "request": "scrollEvent",
      "scrollSpec": {
        "verticalBoundaries": [0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
      },
      "vars": {
        "eventAction": "Percentage",
        "eventCategory": "Scroll Depth",
        "eventLabel": "${verticalScrollBoundary}%",
        "eventValue": 1,
        "ni": true
      }
    },
    "scrollPixel": {
      "on": "scroll",
      "request": "scrollEvent",
      "scrollSpec": {
        "verticalBoundaries": [0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
      },
      "vars": {
        "eventAction": "Pixel Depth",
        "eventCategory": "Scroll Depth",
        "eventLabel": "${scrollTop}",
        "eventValue": 1,
        "ni": true
      }
    },
    "scrollTiming": {
      "on": "scroll",
      "request": "scrollTiming",
      "scrollSpec": {
        "verticalBoundaries": [0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
      },
      "vars": {
        "timingCategory": "Scroll Depth",
        "timingLabel": "${verticalScrollBoundary}%",
        "timingValue": "${totalEngagedTime}000",
        "timingVar": "Percentage"
      }
    }
  }
}