Design application for Xojo

Anybody know of any app which allows for a graphical design of software showing data flows, methods, properties and controls etc?

I already use mind mapping software called NovaMind Pro.

But when working on large projects things become extremely complex and difficult to follow.

I a using EdrawMax Pro for a lot of design work. Has a wealth of drawing and mind mapping features

Look for an Architecture solution like Enterprise Architect. There you can create models for every tier of your application. If you want a simpel solution use MS Visio. Visio allows for wireframes too.