Is it possible to place an item infront of other in FrameworkElement

You could try to add below ZIndex property for a custom FrameworkElement:

public int ZIndex  
        {  
            get  
            {  
                return Canvas.GetZIndex(this);  
            }  
            set  
            {  
                Canvas.SetZIndex(this, value);  
            }  
        }  

Then assign int value to ZIndex when you create it.

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I've created a Zline class like this:

class ZLine : FrameworkElement  
{  
    Line line;  
    public double X1 { get => line.X1; set => line.X1 = value; }  
    public double X2 { get => line.X2; set => line.X2 = value; }  
    public double Y1 { get => line.Y1; set => line.Y1 = value; }  
    public double Y2 { get => line.Y2; set => line.Y2 = value; }  
    public Brush Stroke { get => line.Stroke; set => line.Stroke = value; }  
    public double StrokeThickness { get => line.StrokeThickness; set => line.StrokeThickness = value; }  
    public int ZIndex { get => Canvas.GetZIndex(this); set => Canvas.SetZIndex(this, value); }  

    public ZLine() {  
        line = new Line();  
        AddVisualChild(line);  
    }  
    protected override Size MeasureOverride(Size availableSize) {  
        line.Measure(availableSize);  
        return line.DesiredSize;  
    }  
    protected override Size ArrangeOverride(Size finalSize) {  
        line.Arrange(new Rect(line.DesiredSize));  
        return finalSize;  
    }  
    protected override Visual GetVisualChild(int index) => line;  
    protected override int VisualChildrenCount => 1;  
}  

and added this

public int ZIndex { get => Canvas.GetZIndex(this); set => Canvas.SetZIndex(this, value); }  

property in BarWithBorder class. In the BarChart class in that link, I've modified the addLine method to this:

void addLine() {  
    var line = new ZLine() {  
        StrokeThickness = 1,  
        Stroke = Brushes.LightBlue,  
        IsHitTestVisible = false  
    };  
    Children.Add(line);  
    line.ZIndex = 1;  
    line.Loaded += animateLines;  
}  

and modified the onSourceChanged to set ZIndex of BarWithBorder like this:

 static void onSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {  
     var o = d as Barchart;  
     if (e.OldValue != null) o.Children.Clear();  
     o.labelDesired = new Size(0, 0);  
     o.maxValue = 0;  
     var list = (IEnumerable<int>)e.NewValue;  
     foreach (int value in list) {  
         if(o.IsWithRect) o.Children.Add(new BarWithRect(value));  
         else {  
             var bar = new BarWithBorder(value);  
             Children.Add(bar);  
             bar.ZIndex = 2;  
         }  
         o.addLabel(DateTime.Today.ToString("dd MMM yyyy"));  
         if (value > o.maxValue) o.maxValue = value;  
         if (value < o.minValue) o.minValue = value;  
     }  
     var min = o.minValue < 0 ? o.minValue : 0;  
     double step = 0d;  
     var current = min;  
     if (min < 0) step = (o.maxValue + Math.Abs(min)) / 9;  
     else step = o.maxValue / 9;  
     for (int i = 0; i < 10; i++) {  
         o.addLine();  
         o.addTick(current);  
         current += step;  
     }  
 }  

and changed these lines of ArrangeOverride

else if (item is Line) {  
    var line = item as Line;  
    ....  
}  

to these:

else if (item is ZLine) {  
    var line = item as ZLine;  
    ....  
}  

BUT nothing changed. Bars are still behind the Lines.