These iterm image extensions are almost there. It is unfortunate that it can only display files, and not image created on the fly (i.e., you dump all the pixel values into the terminal and it shows an image).
The iterm image support is pretty simple (just a VT100-style command wrapped around a base64 encoded png). Maybe somebody can implement it for gnome/vte or something, and see if there's any traction.
And, while not quite the same thing, if you are a user of iTerm2, it includes the script "imgcat" that will display an image directly in the terminal window.
Using gnuplot is a nice idea, but the protocol doesn't play nice with other terminals that don't support inline images. See what iTerm2 is doing along these lines. http://www.iterm2.com/images.html
Aside: Although iTerm2 (on macOS) doesn't appear to work with this, iTerm2 actually supports full-color inline images, and has an "imgls" script which does something similar. (There's also "imgcat".)
FWIW, it might be worth mentioning in this context that iTerm 2 also has its own protocol for displaying pixels, so you can cat images in the terminal using an "imgcat" script that they offer for download:
Shame, the idea is pretty neat. If you can solve that issue, OP, I'd love to give it a proper try.
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