Hi David,

Please accept my apology for the delay in responding. 

I have identified one way to achieve the desired outcome that may be suitable for your scenario. A runnable sample is available at: 

http://dojo.telerik.com/UZuNo

To achieve the requirement of visualising aggregates of other value fields in the Kendo UI Chart, use the Kendo UI Chart tooltip sharedTemplate instead of the template configuration property. It allows better control and most importantly, it reveals the original dataItem from the Kendo UI Data Source,thus providing complete freedom for custom calculations.

http://docs.telerik.com/kendo-ui/controls/charts/features/tooltip#templates

Should this not be suitable for your scenario, please refer to my answer in the below forum thread:

http://www.telerik.com/forums/second-series-summed-and-template-dataitem#hAf2eSqhAkSEXyAgwrCTKA

Alternatively, you could take advantage of our tailor-made solution specialists by contacting them at:

http://www.telerik.com/professional-services

Regards,
Alex
Telerik by Progress