the dataitem did not pass to client side if dataitem is not in X/Y.
datasource {"Vendor", "AppName", "Qty", "Date"}
Line chart: Xasix (Date), Yasix(Qty)
<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="100%" Height="400px">
<ClientEvents OnSeriesClick="RadHtmlChart1_OnSeriesClick" />
</telerik:RadHtmlChart>
function RadHtmlChart1_OnSeriesClick(e) {
alert(e.dataItem['Vendor']); // return undefined
}