XAML:
<schementi:Sparkline x:Name="MySparkline" />
Add values:
this.MySparkline.AddTimeValue(double);
The implementation is very basic; Sparkline.AddTimeValue constructs a point at the next time interval and adds it to a Polyline. You can control everything about the line's visuals, including adding visible points along the line and showing horizontal lines for the latest/highest/lowest values.
The source builds assemblies for both .NET 4.0 and Silverlight 4.