Customize color of individual data in LiveChart

Question

Is possible and how to customize individual data color in LiveChart? Instead of having the same color scheme for a series in code behind, I like to programmatically and conditionally color an individual data column. I thought of, as a work around, to separate and overlay the series like having a Red Series {null,null,null,7.8,null} and a Green Series {2,3,2,null,4} but it is strongly typed and I cannot pass in null.

In short, how to I from left to the right:

[EDIT 1]

I read the suggested link but the mapper seems not working properly on LineSeries.

cvValues = new ChartValues<double>() { 1.01, 2.02, 3.03, 4.04, 1.01, 2.02, 3.03, 4.04, 1.01, 2.02 };
cvUSL = new ChartValues<double>() { 3.5, 3.5, 3.5, 3.5, 3.5, 3.5, 3.5, 3.5, 3.5, 3.5 };
cvLSL = new ChartValues<double>() { 1.5, 1.5, 1.5, 1.5, 1.5, 1.5, 1.5, 1.5, 1.5, 1.5 };

DangerBrush = new SolidColorBrush(Colors.Red);

mapper1 = new CartesianMapper<double>()
        .X((value, index) => index + 1)
        .Y((value, index) => value)
        .Fill((value, index) => value > 3.5 ? DangerBrush : null)
        .Stroke((value, index) => value > 3.5 ? DangerBrush : null)
        .Fill((value, index) => value < 1.5 ? DangerBrush : null)
        .Stroke((value, index) => value < 1.5 ? DangerBrush : null);

DataContext = this; 

And here is my XAML (simply wanting the fill in red if a data goes out of upper or lower boundary)

<lvc:LineSeries x:Name="lsUSL"
                Values="{Binding cvUSL}" 
                PointGeometrySize="0"
                PointForeground="White"
                Stroke = "Turquoise"
                StrokeDashArray = "8"
                StrokeThickness = "0.8"
                Fill = "Transparent"
                Configuration="{Binding mapper1}"/>

<lvc:LineSeries x:Name="lsLSL"
                Values="{Binding cvLSL}" 
                PointGeometrySize="0"
                PointForeground="White"
                Stroke = "Turquoise"
                StrokeDashArray = "8"
                StrokeThickness = "0.8"
                Fill = "Transparent"
                Configuration="{Binding mapper1}"/>

<lvc:LineSeries x:Name="lsValues"
                Values="{Binding cvValues}" 
                PointGeometrySize="5"
                PointForeground="Green"
                Stroke = "Gray"
                StrokeThickness = "1"
                Fill = "Transparent"
                Configuration="{Binding mapper1}"/>