You are not defining the Legends in the html.
<asp:Chart ID="RateChart" runat="server" Height="500px" Width="900px">
<Series>
<asp:Series Name="Series1" ChartType="Line">
</asp:Series>
</Series>
<ChartAreas>
<asp:ChartArea Name="ChartArea1">
</asp:ChartArea>
</ChartAreas>
</asp:Chart>
So the error.
To avoid the error add the Legends in th ehtml or add it from code behind.
Defining Legends in HTML
<asp:Chart ID="RateChart" runat="server" Height="300px" Width="400px">
<Titles>
<asp:Title ShadowOffset="3" Name="Items" />
</Titles>
<Legends>
<asp:Legend Alignment="Center" Docking="Bottom" IsTextAutoFit="False" Name="Default"
LegendStyle="Row" />
</Legends>
<ChartAreas>
<asp:ChartArea Name="ChartArea1" BorderWidth="0" />
</ChartAreas>
</asp:Chart>
Defining Legends in Code
RateChart.Legends.Add("Default")
RateChart.Legends(0).Enabled = True