I don’t know how you have set up your diagram nor what result you got that is not what you expected.
But I suspect that the size of your viewport, the HTML DIV element, is large enough to show both nodes, so the diagram is automatically scrolled to show everything. You can confirm this by looking at the value of Diagram.position.
This is similar to how a browser will not scroll a short HTML page up so that its first lines are clipped off the top when there is room to show the whole document in the window.
Read more at http://gojs.net/latest/intro/viewport.html and http://gojs.net/latest/intro/initialView.html.