There are actually a number of more-or-less node/graph-based image editors:

# chaiNNer

Fully node-based image processing.

https://github.com/chaiNNer-org/chaiNNer

# vkdt

Node-based raw and video editor. Sort of the evolution of darktable, by the original developer.

https://jo.dreggn.org/vkdt/readme.html

# darktable

The graph is strictly linear (single input, single output), but you can change the order of the processing and insert new nodes as you want.

https://www.darktable.org

Pretty sure there are some others, but those are the three I can remember right now.