I am doing INITIAL research for software for implementing GUI on Linux /Ubuntu using C++.

Must be open source , free to use , with decent support forum.
I am looking for implementing graphs - not a "space invaders games".

By "decent" I mean no psychoanalysis, lessons on English grammar , how to be nice , put-downs etc.

And most of all - provide technical support and not badges . atta-boys, etc,

Candidates so far - "X11", OpenGL, GTK.

Help me to decide a "product".
Any comments are welcome.

How closely have you looked at Qt for that? It supports C++ natively and is rather easy to figure out.

To be honest I don't get the final design relationship between these. "I am looking for implementing graphs" "implementing GUI on Linux /Ubuntu using C++"

The entire post confused me but I had the same understanding as Turbocapitalist: which toolset.

These are 3 different things.

X11(deprecated) Using xorg now, moving towards wayland.
https://en.wikipedia.org/wiki/X11
Was a graphical environment. So take a look at xorg and wayland.

OpenGL
https://en.wikipedia.org/wiki/OpenGL

GTK
Gnome tool kit. Gnome uses it, so does a lot of software.

There are several toolkits for developing graphical programs for *nix. GTK QT FLTX TK, then there are python bindings for toolkits like PyQt5. There are GUI builders like Glade for gtk, QT desiger for QT, wxDesigner for wxWidgets, gtkmm C++ bindings for gtk. That's not an easy answer quick answer. Depends on what you are doing,

Another one.
https://www.ultimatepp.org/

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It is not too confusing if you read the whole sentence and do not mix two of them together which may be too much to analyse, or read into it , at once.
So for clarification - I want to display graphs , not "shoot them up, blow them up graphics".

Does Qt meets the spec?

" Must be open source , free to use AKA no purchase necessary , with decent support forum"

If you're planning to create your own software for handling graphs, then I expect you could do something with either Qt or GTK. But you may find that the work has been done for you. Even if an existing program is not what you want, it can obviously be re-written or some of its procedures canabalised.
https://linuxappfinder.com/scientificandengineering

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Do you mean some program like these for use in linux? https://www.linuxlinks.com/plottingtools/

"A good plotting tool is very important for generating professional looking graphics for inclusion in academic papers."

Good link, however , I do not intend to argue about "plotting" , "graphs" and "graphics" terminology,
Terminology is important, but so is my objective - so let's settle on "plotting graphs".

Many thanks for everybody's input.

SOLVED I'll will use OpenGL.

That's funny, Gnome wishes it was...Alas: GIMP toolkit https://www.gtk.org/

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