Hello Herve,

Customizing the default tooltip of the RadCartesianChart can be tricky but not impossible. You can try to create a custom chart renderer. This will allow you to access in Xamarin the native charting component. Once you have access to it you will be able to change its ContentAdapter. This adapter is responsible for drawing the content of the tooltip. To do this you will need to create custom ContentAdapter and use it instead of the default one.

In short the steps that you need to do are as follows:
1. Create custom renderer
2. Access the ChartTooltipBehavior of the native charting component
3. Create custom content adapter
4. Use your adapter instead of the default one
5. Create the View that you need to be displayed as tooptip and return it

This approach except the last step is demonstrated in the attached project.

Please take a look at it and let us know if you need any further assistance.

Regards,
Pavel R. Pavlov
Telerik