function divideBySeries (divisorMetricName) {
  return function _divideBySeries (metricInstances) {
    const divisorSeries = metricInstances.find(mi => mi.metric === divisorMetricName).data
    const outputs = metricInstances.filter(mi => mi.metric !== divisorMetricName)
    let result = outputs.map(mi => {
      return {
        ...mi,
        data: mi.data.map(({ ts, value }) => {
          // look up ts in divisor array
          let divPoint = divisorSeries.find(divisor => ts.getTime() === divisor.ts.getTime())
          // return value divided by divisor value
          return { ts, value: value / divPoint.value }
        })
      }
    })
    return result
  }
}
Divide all values (excluding seriesName) by the values in seriesName, assumes there is only one instance
for the divisor metric.

@param {string} metricName the divisor metric name
@return {function} a transform function