The map files are text files which can be opened and edited. I actually used redblobgames' tutorial on hex grids, it's an amazing resource. What type of data format would you like to see?
I use an own human-readable format, but I could add a JSON or YAML export, if someone needs it and has a use case for it. I've uploaded the demo map Hexxmap comes with as an example.
Honestly for me any format would be fine, as long as the spec is clear and makes sense. Knowing there's easily readable map files using hex coords is already good enough. Originally I wasn't sure if there was anything available as the product page only mentioned PNG exports.
