It looks like the binding on your LineSeries is fine, but the WPF-Toolkit LineSeries has a flag IsSelectionEnabled
which is false by default. If you set this flag then you will see the binding working. Add IsSelectionEnabled="True"
to your LineSeries
Try this:
<chartingToolkit:Chart>
<chartingToolkit:LineSeries DependentValuePath="Status"
IndependentValuePath="DateTaken"
ItemsSource="{Binding OverallStatus}"
SelectedItem="{Binding SelectedTrendPoint, Mode=TwoWay}"
IsSelectionEnabled="True"/>
</chartingToolkit:Chart>