docs: function groupSeries(ecModel) {
      var seriesGroupByCategoryAxis = {};
      var otherSeries = [];
      var meta = [];
      ecModel.eachRawSeries(function (seriesModel) {
          var coordSys = seriesModel.coordinateSystem;
  
          if (coordSys && (coordSys.type === 'cartesian2d' || coordSys.type === 'polar')) {
              var baseAxis = coordSys.getBaseAxis();
              if (baseAxis.type === 'category') {
                  var key = baseAxis.dim + '_' + baseAxis.index;
                  if (!seriesGroupByCategoryAxis[key]) {
                      seriesGroupByCategoryAxis[key] = {
                          categoryAxis: baseAxis,
                          valueAxis: coordSys.getOtherAxis(baseAxis),
                          series: []
                      };
                      meta.push({
                          axisDim: baseAxis.dim,
                          axisIndex: baseAxis.index
                      });
                  }
                  seriesGroupByCategoryAxis[key].series.push(seriesModel);
              }
              else {
                  otherSeries.push(seriesModel);
              }
          }
          else {
              otherSeries.push(seriesModel);
          }
      });
  
      return {
          seriesGroupByCategoryAxis: seriesGroupByCategoryAxis,
          other: otherSeries,
          meta: meta
      };
  }