You can show common legend for several charts if you are using Multi-Series Charts as they share one and the same Chart Area. This is described in our help article - http://www.telerik.com/help/wpf/radchart-features-multi-series-charts.html
Have in mind that ChartArea has property Legend and you can set it with instance of the Legend where you want your items to be shown. This could be very useful if you have two Chart Areas as their Legend property can be set with the same Legend instance so that their items will be shown in common Legend.
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