The data is stored in a custom binary file format. Parts of the format are zlib deflated. There is documentation for the format, but it is wrong and outdated. I had to reverse engineer the format from the C++ code.
Issue created: https://github.com/aardappel/treesheets/issues/185
My (partial) parser in Python: https://gist.github.com/quag/e219f69670cd395d4a59a392557df28...
If people would like the current format documented and simple test files to parse please let me know. It’s a 1-2 day task to get it to the point were I wouldn’t have had to read the code to write a parser.