The ClipToPlotArea property determines if the series elements should be clipped within the plot area of the chart. In your case, the data point visual is going outside of the RadCartesianChart control itself. To avoid the clipping in this case, set the ClipToBounds property of the RadCartesianChart control to False. Also, you can add a small Margin to the chart, in order to ensure that there is enough space around the chart.
<telerik:RadCartesianChart ClipToBounds="False" Margin="10">
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