With 2013 Q1 release we introduced a new HoverMode property for the RadChartView. You can use this property to achieve the described scenario. If you are using assemblies from the official release you must also set LegendSettings for each series. This is because the hover mode was tightly coupled to the new Legend support in the chart and there was a bug in the control - the hover mode worked only when LegendSettings were present. This is now fixed.
Let us know if we can be of further help.
the Telerik team