This works fine for me
In aspx; I have the following code
<chart:chart id="Chart1" runat="server" xmlns:chart="#unknown">
<series>
<chart:series name="Series1"></chart:series>
</series>
<chartareas>
<chart:chartarea name="ChartArea1"></chart:chartarea>
</chartareas>
</chart:chart>
DataTable dt = new DataTable();
dt.Columns.Add(new DataColumn("Value1"));
dt.Columns.Add(new DataColumn("Value2"));
dt.Columns.Add(new DataColumn("Value3"));
for (int i = 1; i <= 5; i++)
{
dt.Rows.Add("Hi" + i, 12*i, "Tooltip"+i);
}
Chart1.Series[0].XValueMember = "Value1";
Chart1.Series[0].YValueMembers = "Value2";
Chart1.Series[0].YValuesPerPoint = 5;
Chart1.Series[0].ChartType = SeriesChartType.Pie;
Chart1.DataSource = dt;
Chart1.DataBind();
for(int cnt=0; cnt<chart1.series[0].points.count;cnt++> {
Chart1.Series[0].Points[cnt].ToolTip = dt.Rows[cnt]["Value3"].ToString();
}