Set some properties. You'll need Chart.ImageStorageMode, Chart.Series, Chart.ChartAreas, and Chart.Legends. My working example looks like this:
<asp:Chart ID="cTrafficByMonth" runat="server" ImageStorageMode="UseImageLocation" ImageLocation="Graph" Width="500" TextAntiAliasingQuality="High" AlternateText="Traffic by Month" AntiAliasing="All" >
<Series>
<asp:Series Name="Humans" ChartType="Line" Color="Red" ToolTip="Human traffic" />
<asp:Series Name="Spiders" ChartType="Line" Color="DarkBlue" ToolTip="Spider traffic" />
<asp:Series Name="Total" ChartType="Line" Color="Green" ToolTip="Total traffic" />
</Series>
<ChartAreas>
<asp:ChartArea Name="ChartArea1">
<AxisY Title="Distinct Sessions" />
<AxisX LabelAutoFitStyle="LabelsAngleStep90" />
</asp:ChartArea>
</ChartAreas>
<Legends>
<asp:Legend Docking="Bottom" Alignment="Center" />
</Legends>
</asp:Chart>
And of course you'll need to put some actual data into your chart. You'll need XYZ Coordinates, where XY appear on the chart and Z is your axis number (which I arbitrarily decided equals a Z coordinate). These must correspond with the <asp:Series... /> lines you added in your ASPX.
using (DataTable dt = utils.GetMonthlyHits()) {
dgHits.DataSource = dt;
dgHits.DataBind();
for (int i = 0; i < dt.Rows.Count; i++) {
cTrafficByMonth.Compression = 100;
cTrafficByMonth.Series["Spiders"].Points.AddXY(i, dt.Rows[i]["Spiders"].ToString());
cTrafficByMonth.Series["Humans"].Points.AddXY(i, dt.Rows[i]["Humans"].ToString());
cTrafficByMonth.Series["Total"].Points.AddXY(i, dt.Rows[i]["Total Hits"].ToString());
cTrafficByMonth.Series[0].Points[i].AxisLabel = DateTime.Parse(dt.Rows[i][0].ToString()).ToString("MM/yy"); }