[I'm curious why I got downvoted above - did I miss something?]
I don't think that needs a citation though. Any GUI is a graphical language for a constrained problem. I could use a generic text language to post this comment to HN, or I can use a specialized graphical language that provides me with this resizable text input box and reply button. It's extremely constrained in this case. If it had GUI elements for doing formatting, or the ability to post comments to other websites, it would be a less constrained graphical language. As you reduce constraints the graphical language would either need the ability to create new concepts or have many additional concepts predefined. So to answer your question about a citation: this very comment box is my citation. It works better than a purely text based input for submitting this comment.
Obviously for a general language you can't predefine all the required concepts which means they need to be user defined. User defining concepts in a graphical language is a difficult task as it requires creating uniquely human recognizable symbols for the new concepts.
You have two ways to get those new human recognizable symbols - you can generate them with an AI, or a human must generate them. AI is nowhere near able to generate symbols for concepts it doesn't understand as it would need to be a true AI with the ability to learn and understand new concepts. Having your graphical language's users define new concepts in effect makes those users into language designers. This is a bigger problem than it sounds as language design is an extremely difficult problem, and I personally don't want to be designing a language when I'm trying to solve a problem as I'll no doubt get the language design wrong if I'm focused more on the problem than the language design.
"Any GUI is a graphical language for a constrained problem."
With that broad a definition (and I don't think it's horribly unreasonable), I agree it doesn't need a citation (even if I think GUIs are overapplied).
I don't think that needs a citation though. Any GUI is a graphical language for a constrained problem. I could use a generic text language to post this comment to HN, or I can use a specialized graphical language that provides me with this resizable text input box and reply button. It's extremely constrained in this case. If it had GUI elements for doing formatting, or the ability to post comments to other websites, it would be a less constrained graphical language. As you reduce constraints the graphical language would either need the ability to create new concepts or have many additional concepts predefined. So to answer your question about a citation: this very comment box is my citation. It works better than a purely text based input for submitting this comment.
Obviously for a general language you can't predefine all the required concepts which means they need to be user defined. User defining concepts in a graphical language is a difficult task as it requires creating uniquely human recognizable symbols for the new concepts.
You have two ways to get those new human recognizable symbols - you can generate them with an AI, or a human must generate them. AI is nowhere near able to generate symbols for concepts it doesn't understand as it would need to be a true AI with the ability to learn and understand new concepts. Having your graphical language's users define new concepts in effect makes those users into language designers. This is a bigger problem than it sounds as language design is an extremely difficult problem, and I personally don't want to be designing a language when I'm trying to solve a problem as I'll no doubt get the language design wrong if I'm focused more on the problem than the language design.