Pie chart code is not working

i dynamically set the value in pie chart

My code as follows

<asp:PieChart ID="PieChart1" runat="server" ChartHeight="300"
ChartWidth="450" ChartTitle="Today's Attendance"

<asp:PieChartValue Category="Present" Data="80" />
<asp:PieChartValue Category="Absent" Data="10" />
<asp:PieChartValue Category="Leave" Data="10" />

when i run the above code shows the empty pie chart.

what is the mistake in my above code