It is a bit of a pain point in the current version of the API, but there are many valid ways to approach augmenting the tooltip, regardless. If you are just looking to apply some additional formatting to the value in the tooltip, you may want to set a DataPointTemplate for the series, http://help.infragistics.com/NetAdvantage/WPF/2010.2/CLR4.0/?page=xamChart_Using_a_DataTemplate_to_Style_DataPoints.html

This will allow you to specify a value converter when you bind the Tooltip from the DataPointTemplate to the tooltip somewhere on that visual. see: http://community.infragistics.com/forums/p/40864/229969.aspx#229969

You also have other options in terms of augmenting the data being bound to the chart in order to use a simple DataMapping to define the population of the tooltips see the two methods shown below:

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            DataTable testTable = GetTestData();
            CreateSeriesLinq(testTable);
        }

        private void CreateSeriesExtraColumns(DataTable testTable)
        {
            int columnCount = testTable.Columns.Count;
            for (int i = 0; i < columnCount; i++)
            {
                DataColumn newColumn = new DataColumn(
                    testTable.Columns[i].ColumnName + "ToolTip");
                testTable.Columns.Add(newColumn);
                foreach (DataRow row in testTable.Rows)
                {
                    row[newColumn] = 
                        testTable.Columns[i].ColumnName + " = " + row[i];
                }
            }

            for (int i = 0; i < columnCount; i++)
            {
                Series seriesSales = new Series();
                seriesSales.ChartType = ChartType.Column;
                seriesSales.DataSource = testTable;
                seriesSales.DataMapping = "Value=" + 
                    testTable.Columns[i].ColumnName + ";Tooltip=" + 
                    testTable.Columns[i + columnCount].ColumnName;
                seriesSales.Label = testTable.Columns[i].ColumnName;
                seriesSales.StrokeThickness = .5d;
                theChart.Series.Add(seriesSales);
            }
        }

        private void CreateSeriesLinq(DataTable testTable)
        {
            int columnCount = testTable.Columns.Count;
            for (int i = 0; i < columnCount; i++)
            {
                Series seriesSales = new Series();
                seriesSales.ChartType = ChartType.Column;

                int currIndex = i;
                seriesSales.DataSource = 
                    from row in testTable.AsEnumerable()
                    select new 
                    { 
                        Value = row[testTable.Columns[currIndex]],
                        Tooltip = testTable.Columns[currIndex].ColumnName +
                        " = " + 
                        row[testTable.Columns[currIndex]] 
                    };
                seriesSales.DataMapping = "Value=Value;Tooltip=Tooltip";
                seriesSales.Label = testTable.Columns[i].ColumnName;
                seriesSales.StrokeThickness = .5d;
                theChart.Series.Add(seriesSales);
            }
        }

        private DataTable GetTestData()
        {
            DataTable testTable = new DataTable("TestTable");

            testTable.Columns.Add(new DataColumn("NumberCars"));
            testTable.Columns.Add(new DataColumn("NumberPlanes"));

            DataRow row = testTable.NewRow();
            row["NumberCars"] = 54;
            row["NumberPlanes"] = 26;
            testTable.Rows.Add(row);

            row = testTable.NewRow();
            row["NumberCars"] = 21;
            row["NumberPlanes"] = 6;
            testTable.Rows.Add(row);

            row = testTable.NewRow();
            row["NumberCars"] = 12;
            row["NumberPlanes"] = 31;
            testTable.Rows.Add(row);

            row = testTable.NewRow();
            row["NumberCars"] = 63;
            row["NumberPlanes"] = 1;
            testTable.Rows.Add(row);

            row = testTable.NewRow();
            row["NumberCars"] = 15;
            row["NumberPlanes"] = 11;
            testTable.Rows.Add(row);

            row = testTable.NewRow();
            row["NumberCars"] = 3;
            row["NumberPlanes"] = 52;
            testTable.Rows.Add(row);

            row = testTable.NewRow();
            row["NumberCars"] = 26;
            row["NumberPlanes"] = 54;
            testTable.Rows.Add(row);

            return testTable;
        }
    }

There are other ways of approaching this also. If you have a way you would prefer the API worked, also, please go ahead and make a feature request and hopefully we will be able to accomodate soon. Hope this helps!

-Graham

Doing something like this can give you more control over what gets displayed in the tooltip and how: http://forums.infragistics.com/forums/p/38424/221145.aspx#221145

If you do that then you should be able to throw in some value converters into the tooltip template, etc. Let me know if you need additional direction for this appoach. Hope this helps!

-Graham