The current version of RadChartView does not support such customization and it would not be possible to achieve the desired effect. We are currently working on a new ChartView feature for Q3 2012 (annotations support) and we will forward your feedback to our developers so they can consider enabling this scenario as part of the annotation effort as well (e.g. provide public methods for conversion between physical pixels and data units that can be used to achieve this functionality).
If this requirement is critical to your project, you might consider using the RadChart control instead -- see the attached sample application that demonstrates what-if scenario for Line series.
the Telerik team