private void UpdatePolygon(ref Polygon poly, int value, int posX, int posY, int maxHeight, int historyLength = 30)
{
    int v = 100 / maxHeight;
    value = value == 0 ? value : value / v;
    value = value * -1; // expand up

    int length = 0;
    System.Windows.Point end = new System.Windows.Point(historyLength + posX + 1, posY);

    // Create a blue and black Brush
    SolidColorBrush whiteBrush = new SolidColorBrush();
    whiteBrush.Color = Colors.White;
    SolidColorBrush blackBrush = new SolidColorBrush();
    blackBrush.Color = Colors.Black;

    // Create a Polygon
    if (poly == null)
    {
        PointCollection polygonPoints = new PointCollection();

        // Create a collection of points for a polygon
        System.Windows.Point Point1 = new System.Windows.Point(posX + 3, posY);
        poly = new Polygon();
        poly.Stroke = whiteBrush;

        poly.StrokeThickness = 0.5;
        poly.Points.Add(end);

        polygonPoints.Add(Point1);

        for (int i = 0; i < historyLength; i++)
        {
            polygonPoints.Add(new System.Windows.Point(i + posX + 3, posY));
        }

        // Set Polygon.Points properties
        poly.Points = polygonPoints;
        poly.Name = "poly";

        // Add Polygon to the page
        grid.Children.Add(poly);
        poly.Fill = whiteBrush;
    }

    length = poly.Points.Count;

    poly.Points.RemoveAt(1);

    for (int i = 1; i < historyLength - 1; i++)
    {
        System.Windows.Point p = poly.Points[i];
        p.X -= 1;
        poly.Points[i] = p;
    }

    poly.Points.RemoveAt(length - 2);
    poly.Points.Add(new System.Windows.Point((length) + posX, value + posY));
    poly.Points.Add(end);
}