Using method from Diagram.Panel in ViewModel

I am trying to add nodes to my diagram. My View snippet is as follows:
<go:Diagram x:Name=“Diagram”
Model="{Binding Path=Model, Mode=TwoWay}"
NodeTemplate="{StaticResource ResourceKey=NodeTemplate}"
DataContext="{Binding DiagramViewModel}"
SelectedPart="{Binding SelectedPart, Mode=TwoWay}"/>

DiagramViewModel is a property in the main view model. I have a button which adds new nodes.
When that button is clicked, I want to use Diagram.Panel.ComputeBounds method to compute the rectangle of the diagram, add an offset to the rect bounds and use that to set the location of the newly added node. But to do this, I would require Panel property in ViewModel which has a private set.

The arrangement of Nodes should be the responsibility of Diagram.Layout.
Perhaps you want to set Diagram.Layout to an instance of GridLayout.

I want to manually handle the layout.

You can implement a custom DiagramLayout that does whatever you want.

Can you provide a sample if possible?

Please read GoXam for WPF 2.2.4 and override GoXam for WPF 2.2.4

For examples, search the samples for subclasses of DiagramLayout. Maybe I can dig one up for you.