Custom tooltips

Hi,

 I have a SplineChart displaying a monthly number of customers. Now I'd like to have a tool-tip showing for each month the increase or decrease of customers since previous month. 

I've seen this page : http://demos.telerik.com/aspnet-ajax/htmlchart/examples/functionality/clienttemplates/defaultcs.aspx, but since my telerik version is outdated (2012.3.1016.35)  I can't use <ClientTemplate> token inside of  <TooltipsAppearance>  </TooltipsAppearance>.

Instead I used :
 <TooltipsAppearance  ClientTemplate = "#=dataItem.MonthlyIncrease#"    BackgroundColor="Red"> </TooltipsAppearance>

Where MonthlyIncrease is a column of the dataTable binded to my chart. However this isn't working, the tooltip only show the monthly number of customers, not the increase (and the background is still white). 

Thank you very much for your help,
Pierre. 

Bellow more of my code:

 <telerik:RadHtmlChart runat="server" ID ="SplineChartVehiculesSixMois" Skin="Simple" Transistions = "true" ToolTip = "true">

    <PlotArea>

        <Series>                                     

            <telerik:BarSeries>                                         

                <TooltipsAppearance  ClientTemplate = "#=dataItem.NombreVehi#" BackgroundColor="Red">  

                </TooltipsAppearance>

            </telerik:BarSeries>

         </Series>

     </PlotArea> 

</telerik:RadHtmlChart>