I need to position easy objects, layout in a nice way, and export into JSON. Do you know something like this? Omnigraffle unfortunately not support JSON export.
Can you specify an example use case? That would help give a better answer.
It sounds like vector graphics may be useful for you. Try looking at Adobe Illustrator or a similar program that exports svg.
From Wikipedia: SVG images and their behaviors are defined in XML text files. This means that they can be searched, indexed, scripted, and compressed. As XML files, SVG images can be created and edited with any text editor, as well as with drawing software.
If you truly need json formatting, you may then convert using svgson or a web-based converter.