WPF Ellipse size - wpf

Question

I have found this question here: How to populate a WPF grid based on a 2-dimensional array and I wanted to use that the marked solution. And it also seems to work quite nice but there is one problem and that problem is that I am using ellipses to display my data. So every item equals one ellipse.
And here is the problem: The ellipses are requesting on Measure only a size of x = 0, y = 0.
So my problem is that there is displayed nothing. So I tried to create a custom Stackpanel:
public class StretchStackPanel : StackPanel
{
protected override Size MeasureOverride(Size constraint)
{
Size result = constraint;
result.Width = result.Width == Double.PositiveInfinity ? Double.MaxValue : result.Width;
result.Height = result.Height == Double.PositiveInfinity ? Double.MaxValue : result.Height;
if (this.Orientation == System.Windows.Controls.Orientation.Horizontal)
result.Height = result.Width / InternalChildren.Count;
return result;
}
protected override Size ArrangeOverride(Size arrangeSize)
{
var size = base.ArrangeOverride(arrangeSize);
double elementLength;
if (Orientation == System.Windows.Controls.Orientation.Horizontal)
elementLength = size.Width / InternalChildren.Count;
else
elementLength = size.Height / InternalChildren.Count;
double current = 0;
foreach (UIElement e in InternalChildren)
{
Rect result;
if (Orientation == System.Windows.Controls.Orientation.Horizontal)
{
result = new Rect(current, 0, elementLength, size.Height);
current += result.Width;
}
else
{
result = new Rect(0, current, size.Width, elementLength);
current += result.Height;
}
e.Arrange(result);
}
return size;
}
}
But there is a very strange behaviour. There are very very smalle margins between the items and the seem to be random. If I resize you can see them moving.
here are two screenshots:
I also tried to use a Uniformgrid with columns only instead of Stackpanel and set an itemstyle with a fixed height using a ViewBox around the Itemscontrol.