Depending on your scenario you can use the MarkerShape property of the legend items to select a shape. This you can do during the DataBound event. I have attached a simple app that demonstrates this.
This approach would work if it is possible to determine what shape should the legend marker be, meaning if it is possible to extract this info from, say, the series mapping (in the attached example - the series definition).
Let us know if this solution would work for your current project.
the Telerik team
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