getStackedSeriesTotalsByGroups() {
    const w = this.w
    let total = []

    w.globals.seriesGroups.forEach((sg) => {
      let includedIndexes = []
      w.config.series.forEach((s, si) => {
        if (sg.indexOf(w.globals.seriesNames[si]) > -1) {
          includedIndexes.push(si)
        }
      })

      const excludedIndices = w.globals.series
        .map((_, fi) => (includedIndexes.indexOf(fi) === -1 ? fi : -1))
        .filter((f) => f !== -1)

      total.push(this.getStackedSeriesTotals(excludedIndices))
    })
    return total
  }
@memberof CoreUtils
returns the sum of values in a multiple stacked grouped charts
Eg. w.globals.series = [[32,33,43,12], [2,3,5,1], [43, 23, 34, 22]]
series 1 and 2 are in a group, while series 3 is in another group
 @return [[34, 36, 48, 12], [43, 23, 34, 22]]