Hi Olivier Vayssie,
Thanks for choosing Syncfusion Products.
You can format the segment label content by writing data template with the converter concept.
You can set the custom label template using the below code.
XAML:
C#:
this.Chart1.Areas[0].Series[0].AdornmentsInfo.LabelTemplate = this.Resources["AdornmentTemplate"] as DataTemplate;
Please download the updated sample from the following location.
http://help.syncfusion.com/support/Chart.WPF/7.4.0.20/F92026/main.htmLet us know if you have any questions.
Thanks & Regards,
Murugavel P