Thank you for your response.
As my colleague Patrick is away from the office I am going to pick this up.
I see from running your application that the line annotation maintains its x position during pinch and pan gestures, which is the expected behaviour.
I appreciate that your issue relates to the programmatic setting of the x position of the annotation. Whilst I am not experienced in C#, I gather from looking at your code that you set a timer, which at a 1 second interval updates the X position of the annotation and asks to redraw the chart. As such I am making the assumption that when running your application, the expected behaviour is to see the line annotation move, without needing to make any user gestures on the device. If my understanding of this is incorrect please let me know!
As my colleague Patrick has mentioned, you should be able to set the position of an annotation programmatically, in a similar way to how you have done in your code. This morning I wrote a quick application which simply takes our 'Quickstart' application and adds a vertical line annotation. I also added a button which when clicked sets the X value of the annotation and asks the chart to redraw. I observed the line annotation moving along the x axis to the new position.
Whilst I performed this task on a native Android application, I see no reason why this should not also work on Xamarin.
You ask in your question if there is another way to achieve this (programmatically moving the x position of the annotation). I would say that you seem to be already using the correct approach and as such I would try to (temporarily, for testing purposes) simplify the invocation of the x position setting code to rule out any other issues.
Would it be possible for you to try a simple approach, possibly using a button to set a new x value on your annotation? We currently are not aware of any issues with the setting of x or y positions of our annotations. Having said that, if you strongly feel that this may be a bug, please contact us an firstname.lastname@example.org and we will certainly look into this further for you.
Thanks and kind regards,