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http://www.telerik.com/help/windows-phone/radgauge-getting-started.html
says
xmlns:gauges="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls.DataVisualizations"
should be
xmlns:gauges="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls.DataVisualization"
says
xmlns:gauges="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls.DataVisualizations"
should be
xmlns:gauges="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls.DataVisualization"