The Step Line chart type is currently not supported. However, this is a quite reasonable request and therefore I have it in our Public Issue Tracking System - PITS. You can track its progress, subscribe for status changes and add your vote/comment to it on the following link - PITS item.
You can find more information about custom rendering in RadChartView in the referred help article.
I hope this information helps. Should you have further questions, I would be glad to help.
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