dataviz: was a common xml namespace alias used for the silverlight toolkit data visualisation namespce. This was for Silverlight 3 version at the time when libraries weren't able to define their own XML namespaces.
The .NET namespace System.Windows.Controls.DataVisualization would be mapped to the prefix dataviz
like this:-
xmlns:dataviz="clr-namespace:System.Windows.Controls.DataVisualization;assembly=System.Windows.Controls.DataVisualization.Toolkit"
xmlns:charting="clr-namespace:System.Windows.Controls.DataVisualization.Charting;assembly=System.Windows.Controls.DataVisualization.Toolkit"
With Silverlight 4 enabling libraries to define their own schema url to cover a whole range of .NET namespaces things changed to this:-
xmlns:toolkit="http://schemas.microsoft.com/winfx/2006/xaml/presentation/toolkit"
This xmlns space covers most objects that you might want to include in Xaml (not just the charting stuff). The older style still works but generally in Silverlight 4 you only need this single alias to refer to anything in you need from the toolkit.