Change controls zero point in canvas - wpf

Question

When i draw items in canvas, the position is take from top of the item inside canvas:
<Canvas Name="cnvMain"
Width="80"
Height="80">
<Ellipse Canvas.Top="20"
Canvas.Left="40"
Width="40"
Height="40"
Fill="Gray" />
</Canvas>
Now what i would need is to take the measurement from the bottom of the element, like this:
In the end i would like to be able to set the distance from the bottom of the Ellipse to the top of the Canvas.
NB: Ellipse can sometimes not be a circle and flipping it upside down is not an option. Also i dont know the height before runtime so setting negative margin in xaml will not work also.
(Picture and example xaml taken from: http://weblogs.asp.net/psheriff/centering-text-within-a-wpf-shape-using-a-canvas)

Answer 1

If you want to specify the distance of the bottom of the Ellipse to the bottom of the Canvas, you could replace Canvas.Top by Canvas.Bottom:
<Ellipse Canvas.Bottom="20" Canvas.Left="40" Width="40" Height="40" Fill="Gray"/>
For the distance from the bottom of the Ellipse to the top of the Canvas, you may specify a negative Margin:
<Ellipse Canvas.Top="20" Canvas.Left="40" Width="40" Height="40" Fill="Gray"
Margin="0,-40,0,0"/>
or an appropriate RenderTransform:
<Ellipse Canvas.Top="20" Canvas.Left="40" Width="40" Height="40" Fill="Gray">
<Ellipse.RenderTransform>
<ScaleTransform ScaleY="-1"/>
</Ellipse.RenderTransform>
</Ellipse>
In order to invert the y direction of the whole Canvas coordinate system, you may apply a RenderTransform to the Canvas:
<Canvas ... RenderTransformOrigin="0,0.5">
<Canvas.RenderTransform>
<ScaleTransform ScaleY="-1"/>
</Canvas.RenderTransform>
...
</Canvas>
Edit: You may also put the Ellipse in another Canvas with zero Height, and attach it to the lower Canvas border:
<Canvas Canvas.Top="20" Canvas.Left="40" Width="0" Height="0">
<Ellipse Canvas.Bottom="0" Width="40" Height="40" Fill="Gray"/>
</Canvas>