mouse hover over tooltip on asp.net chart

I am creating a chart using code-behind. I want to show yvalues on the chart when user hovers the mouse over on the column bar. I looked at other forums and tried the following(first in aspx file and then in the code-behind) but didnt work. what am I missing?
<asp:chart ...> <series> <asp:series name="SampleSeries" xvaluemember="Date" yvaluemember="Value" charttype="Line" tooltip="Date: #VALX{d}" xvaluetype="DateTime" /> </series> ... </asp:chart> chart1.Series[0].ToolTip = "Y Value: #YVAL";