如何格式化highcharts .NET堆栈标签合计

using DotNet.Highcharts;
using DotNet.Highcharts.Enums;
using DotNet.Highcharts.Helpers;
using DotNet.Highcharts.Options;

public class ChartController : Controller
{
    public ActionResult Index()
    {
        // 创建一个Highcharts图表对象
        Highcharts chart = new Highcharts("chart")
            .InitChart(new Chart { Type = ChartTypes.Column })
            .SetTitle(new Title { Text = "堆栈图表" })
            .SetXAxis(new XAxis { Categories = new[] { "类别1", "类别2", "类别3" } })
            .SetYAxis(new YAxis { Title = new YAxisTitle { Text = "值" } })
            .SetSeries(new[]
            {
                new Series { Name = "系列1", Data = new Data(new object[] { 1, 2, 3 }) },
                new Series { Name = "系列2", Data = new Data(new object[] { 4, 5, 6 }) }
            })
            .SetPlotOptions(new PlotOptions
            {
                Column = new PlotOptionsColumn
                {
                    Stacking = Stackings.Normal,
                    DataLabels = new PlotOptionsColumnDataLabels
                    {
                        Enabled = true,
                        Formatter = "function() { return this.y + ' 合计'; }" // 自定义的格式化函数
                    }
                }
            });

        return View(chart);
    }
}