Sorry, the marketing side of me got too excited. Yes it's wxWidgets.
Peeking at the source suggests wxWidgets.
Re:TreeSheets: Free Form Data Organizer
by alexchamberlain on Sunday August 20, 2017 @05:13AM
(#15057624)
Does it support formulas?
The current ability to compute values is not quite the same as formulas in a spreadsheet (which hide "behind" cells), this is meant more as a visual programming language where the operators are part of your data.
Example: http://strlen.com/treesheets/docs/images/screenshots/screens...
But yes, I must admit, that part is very unfinished.
That's a lofty claim, might want to make it more specific to thought mapping.
Other than that looks cool, will try it out when I'm home. What's the GUI framework, out of curiousity?