Please see my comments to the question. You can use either
and GDI+ or WPF, not all together (in fact, you can use interop, but this is not practical).
If you can consider using WPF, I would strongly recommend using it. It is designed with the support of many different activities in mind; and diagramming is one of the major programming chores supported very well. Most things like interaction with the user, vector graphics (with the use of XAML, not as universal as with SVG, but practically the same for diagramming purposes), zoom and pan — all that is already implemented.
Please see my past answers where I tried to sketch the design of diagramming software:
Connect different controls
Vector graphics software like CorelDraw or Inkscape in C# or VB.NET
Architecture diagram tips