code: function groupData(data, fieldIndex) {
      const hashMap = new Map();
      const groupedData = [];
  
      data.forEach((datum) => {
          const fieldVal = datum[fieldIndex];
          if (hashMap.has(fieldVal)) {
              groupedData[hashMap.get(fieldVal)][1].push(datum);
          } else {
              groupedData.push([fieldVal, [datum]]);
              hashMap.set(fieldVal, groupedData.length - 1);
          }
      });
  
      return groupedData;
  }
docs: Groups the data according to the specified target field.
  
  @param {Array} data - The input data array.
  @param {number} fieldIndex - The target field index within schema array.
  @return {Array} Returns an array containing the grouped data.