列の集計

xmlns:ig="clr-namespace:Infragistics.XamarinForms.Controls.Grids;assembly=Infragistics.XF.DataGrid"
...
<ig:XamDataGrid x:Name="DataGrid"
                   GroupHeaderDisplayMode="Combined"
                   SummaryScope="Both"
                   GroupSummaryDisplayMode="RowTop"
                   ItemsSource="{Binding}"
                   AutoGenerateColumns="False">
    <ig:XamDataGrid.Columns>
        <ig:NumericColumn HeaderText="Salary" Width="*" PropertyPath="Salary" FormatString="C" />
        <ig:NumericColumn HeaderText="Sales" PropertyPath="Sales" Width="*" FormatString="C" />
        <ig:TextColumn HeaderText="Name" PropertyPath="Name" HorizontalAlignment="Center"  Width="*" />
    </ig:XamDataGrid.Columns>
    <ig:XamDataGrid.SummaryDescriptions>
        <ig:ColumnSummaryDescription PropertyPath="Salary" ValueFormat="C" Operand="Average"/>
        <ig:ColumnSummaryDescription PropertyPath="Sales" ValueFormat="C" Operand="Max" />
        <ig:ColumnSummaryDescription PropertyPath="Sales" ValueFormat="C" Operand="Min" />
        <ig:ColumnSummaryDescription PropertyPath="Name" Operand="Count"/>
    </ig:XamDataGrid.SummaryDescriptions>
</ig:XamDataGrid>

var salaryAvg = new ColumnSummaryDescription();
salaryAvg.PropertyPath = "Salary";
salaryAvg.Operand = SummaryOperand.Average;
salaryAvg.ValueFormat = "C";

var salesMax = new ColumnSummaryDescription();
salesMax.PropertyPath = "Sales";
salesMax.Operand = SummaryOperand.Max;
salesMax.ValueFormat = "C";

var salesMin = new ColumnSummaryDescription();
salesMin.PropertyPath = "Sales";
salesMin.Operand = SummaryOperand.Min;
salesMin.ValueFormat = "C";

var nameCount = new ColumnSummaryDescription();
nameCount.PropertyPath = "Name";
nameCount.Operand = SummaryOperand.Count;

this.DataGrid.SummaryDescriptions.Add(salaryAvg);
this.DataGrid.SummaryDescriptions.Add(salesMax);
this.DataGrid.SummaryDescriptions.Add(salesMin);
this.DataGrid.SummaryDescriptions.Add(nameCount);

var countries = new ColumnSummaryDescription();
countries.PropertyPath = "Countries";
countries.Operand = SummaryOperand.Custom;
countries.ProvideCalculator += Countries_ProvideCalculator;
this.DataGrid.SummaryDescriptions.Add(salaries);

private void Countries_ProvideCalculator(object sender, ProvideCalculatorEventArgs args)
{
    args.Calculator = new CustomDomestic();
}

class CustomDomestic : SummaryCalculator
    {
        public int countries;
        public override string DisplayName
        {
            get
            {
                return "USA";
            }
        }

        public override void BeginCalculation(IDataSource dataSource, string propertyName)
        {
            base.BeginCalculation(dataSource, propertyName);
            this.countries = 0;
        }

        public override void Aggregate(object item)
        {
            if(item = "USA")
            {
                this.countries++;
            }
        }

        public override ISummaryResult EndCalculation()
        {
            return new DefaultSummaryResult(
                this.PropertyName,
                SummaryOperand.Custom,
                this.countries);
        }
    }