The custom grid lines in RadChart are annotations. Currently there are no annotations in RadChartView, but we are working on this feature for the next official release (in about two months from now). Can you please explain what is the reason you need this draggable line? What are the semantics underneath this? What information will this bring into view, and how will it improve the perception of the data? The reason I am asking is because we are currently working on the annotations and if we decide that the depicted scenario is valid and valuable, we might consider making it an out-of-the-box feature.
Currently the only way I can see this happening is if you place a Line on top of the chart and manage its dragging manually.
the Telerik team