Hi Marc,
Thanks for your interest in Syncfusion products.
We could also write templates and access underlying data source values with tag property. We have created a sample, with StockDataPoint class with IsUp property and applied the same template provided by you.
Please refer to the modified sample in the below given link.
http://files.syncfusion.com/support/Chart.WPF/7.3.0.6/F87886/main.htmPlease let us know if you have further queries.
Regards,
K. Saradha Devi