This is an elisp code grabs a figure from {clipboard| screen selection}, save it under the folder ./drawings, and paste the proper reference to the figure into the currently open {TeX| org} file. When paste from clipboard, the clipboard contains either the image or the path pointing to an image. It runs with Emacs. A typical workflow is as follows, while having xournalpp (or any other drawing app) open:

1. sketch inside xournalpp and copy the sketch
2. switch back to the TeX file and invoke pastel-figure
3. type in figure name after the prompt

xclip, scrot

After copying the sketch into the clipboard, pastel-figure invokes xclip and creates a png file inside ./drawings. It saves the image inside the clipboard into a png file, then refers to the file using inclugraphics at the point of the tex file or bracket reference for org file.

It saves the image into ./drawings folder, and adds reference to the image.

Include the folder that contains past.el. Then load it. Inside init.el you can add the following

(add-to-list 'load-path "path/to/past.el/folder")
(load "past.el")

The default keybinding is the following. You can customize inside past.el:

(,(kbd "H-y") . ,(function pastel-figure))
(,(kbd "H-Y") . ,(function pastel-file))    
(,(kbd "H-o") . ,(function pastel-screenshot-part))

This means, in LaTeX mode, H-y inserts the plot together with the figure environment wrapping, while invoking it with a prefix such as C-- H-i only inserts the line of includegraphics. In org-mode, H-y inserts the link to the figure and toggle inline display automatically.

I tested with xournalpp, but I do not see any problem to substitute this as anything else that copy a figure into the X11 clipboard. To make sure the tool is compatible, copy whatever you want to paste, and check with the following xclip command

xclip -selection clipboard -t TARGETS -o

If you see there is "image/png" in it, it should in principle work.

I appreciate the help from bpalmer, wasamasa, and pjb from #emacs at irc.libera.chat.