I found the answer.
Here is the style for an arbitrary squiggle.
(DVC is my xmlns for the Datavisualistion Charting toolkit).
In my Grid.Resources:
<Style x:Key="Squiggle" TargetType="DVC:ScatterDataPoint">
<Setter Property="Width" Value="18" />
<Setter Property="Height" Value="18" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="DVC:ScatterDataPoint">
<Canvas>
<Path Stroke="Black" StrokeThickness="1"
Data="M 1,20 C 10,30 40,30 40,20 H 30" />
</Canvas>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
The squiggle marker is just an example. You can draw any shape you like:
Shapes and drawing
Then in the Chart.Series:
<DVC:ScatterSeries Title="Squiggles" DataPointStyle="{StaticResource Squiggle}"
IndependentValueBinding="{Binding Path=Key}"
DependentValueBinding="{Binding Path=Value}">
</DVC:ScatterSeries>