It isn’t possible to do this natively, however the LegendControl is just an ItemsControl with DataTemplate that binds to a collection of SeriesInfo objects provided by LegendModifier, so should be easy to create a custom legend.
Take a look at the hierarchy of the LegendControl with a visual debugger, e.g. WPF Snoop. Also check out SeriesInfo in the API documentation so you can see the available properties.
Also take a look at this thread which talks about custom legends and includes the ControlTemplate for the LegendControl.
Hope this helps!
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