interface IgxSummaryResult {
  key: string;
  label: string;
  summaryResult: any;
}

class CustomNumberSummary extends IgxNumberSummaryOperand {
  constructor() {
    super();
  }
  public operate(data: any[], allData: any[], fieldName: string, groupRecord: IGroupByRecord): IgxSummaryResult[] {
    const result = super.operate(data, allData, fieldName, groupRecord);
    result.push({
      key: "avg",
      label: "Avg",
      summaryResult: IgxNumberSummaryOperand.average(data)
    });
    result.push({
      key: 'mdn',
      label: 'Median',
      summaryResult: this.findMedian(data)
    });
    return result;
  }
}
this.grid.getColumnByName('ColumnName').summaries = CustomNumberSummary;

IgxSummaryOperand.average(data);

IgxSummaryOperand.count(dataSource);

IgxNumberSummaryOperand.max(data);

IgxNumberSummaryOperand.min(data);

IgxNumberSummaryOperand.sum(data);