If you want to try it out I enabled it on the beta site. There aren't any docs for the function stuff, but this is how it works:

• Prefix = before a function name. Supported functions: sum, avg, min, max (for numbers), upper, lower, replace (for strings)
• Functions can have args or no args. With args, you can pass expressions or variables (see below). No args means it will try to implicitly get the value(s) from its dependencies. Eg =sum() will sum all its upstream deps.
• You can make variables eg var foo in a node will have that node take on its parents value.
• It supports numbers =1 , =1.1 , strings ="foo" , and basic math expressions.
• Ctrl-Option-E toggles computation which can be helpful to debug

The production version is knotend.com, but computation isn't supported there.

Here's the graph that I did in the demo if you don't want to start from scratch.

That's a really nice way to visualize computation 🙂 My initial thought, and this might me half-baked and fall apart on closer inspection, was that "why do you need separate nodes for variables?". Excel has the concept of named cells, could you use something similar, e.g. there's a naming feature/action on nodes, but naming doesn't introduce additional nodes?