I like Blender's compositor nodes.
It doesn't mean I know how to use them well, but I like the UI and I think it works very well in that context...
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SVG and Canvas example of nodes with zoom and add new nodes features
static.laszlokorte.de/graph/
also check Laszlo implementation of Bret Victor's talk "Drawing Dynamic Visualizations"
github.com/laszlokorte/reform-swift
interactive math expressions visualizer
thetamath.com/
implementation of your mockup
codepen.io/xgundam05/pen/KjqJn
another implementation that looks good
github.com/sketchpunk/NEditorJS
I have been looking for something like this too build interfaces with. Saw a cool video recently for visual programming: [Jonas Gebhardt - Evolving the Visual Programming Environment with React at react-europe 2016]: youtube.com/watch?v=WjJdaDXN5Vs