I'd like to say i really love Diagram designer, i use it alot and i'm very pleased with it.
I have a feature request though
At this time you have the button F6 bound to "Move Canvas", i'd really love this to be the "Space bar" (Would be even more great if pressing spacebar means "Move Canvas" and releasing it returns to previous selected tool). As for zooming in and out, it would be great if you'd bind this to CTRL + and CTRL -, these are button's i use daily in Adobe Photoshop :-)
Also it would be nice when doubleclicking on an object would edit the "Text" of this object
Its not like these features are mandatory but i do believe most people will feel "comfertable" with these buttons, perhaps an option to decide it yourself?
Great work keep it up!