git-cola seems to come with a tool named "git dag" you might wish to take a look at. You can find some screenshots here, let me pick the two most relevant:
git dag (left) and git cola (right), source: GitCola (click images for larger variants)
The two rawly correspond to gitk (git-dag) and git-gui (git-cola), but have a fresher design. You can ...
How about draw.io?
Can be downloaded and used offline as an installed- or as a portable program, or can be used online without an account and with saving files locally.
Canvas automatically expands
You can draw freehand lines
You can import images
Has a huge catalogue of shapes and support of custom shapes
Colors (Fill, Line, Text, Gradients, ...)
I would like to suggest https://github.com/florianschanda/miss_hit :)
There are several tools in this suite, but one of them (mh_style) can auto-format code. I believe this tool fulfils all of your criteria above.
We use and sell 'microsoft PowerApps'. You can make a bunch of different applications with similar code to excel. It will require some knowledge but there are free courses for that as well. If all else fails, paying someone to create a PowerApps should be a whole lot cheaper than paying a programmer to do something from scratch