Hi Luka,
Greetings from Syncfusion, we have analyzed your requirement and we like to let you know that there is no direct support for crosshair in Xamarin Chart however we have achieved your requirement using ChartTrackballBehavior and Annotation in iOS and Android.
Since we have crosshair support in our native UWP we have achieved the same in below sample by mapping the native using CustomRenderer. You can download the sample from the below link
Hope this helps.
Thanks,
Michael