Hi Arnaud,
Thanks for choosing Syncfusion products.
You can achieve the requirement by writing custom FastSeriesPresenter. You can specify different interior brush of the segment in OnRender method.
Please find the simple sample attached which demonstrates the rendering of fast line chart with multiple interiors.
Sample details are as follows.
1) Chart is bound to a model. The series is bound to a collection object of custom data, which has a property called Flag to specify the good / bad value. (Modify this according to your application).
2) OnRender will draw line Segments with different pen for different Flag values.
Let me know any queries you have.
Regards,
Abdul Rahman
ChartSample_2a1ae9b1.zip