Hi, Ian-

I think you will be happier with the full pythonic version, yes! But indeed, we need to fix that error message.

So, the default package manager in Ubuntu 20.04 (Aptitude) distributes a version of Matplotlib, 3.1.2, that contains this error when trying to make JPGs:
[bugs.launchpad.net]
I don't know why they haven't fixed it, and it is the root cause of the error here.

The good news: there is a fix. Can you please run the following on your computer (requires the admin/sudo password):

sudo apt-get install pip
sudo apt-get remove python3-matplotlib
pip install matplotlib

That should get you matplotlib ver=3.6.2 (or so), and Python can now generate JPG images. (Why Ubuntu didn't make the bug-free version of matplotlib its default... I don't know.)

If you want to test this fix without rerunning the full afni_proc.py command, you can do this:
Go into an afni_proc.py (AP) results directory, and do:

curl -O https://raw.githubusercontent.com/afni/afni/master/src/ptaylor/supplement/redo_apqc.tcsh
tcsh redo_apqc.tcsh

... to regenerate the QC directory there. Under the 'mot' block, you should now see lots of line plots.

Please let me know how that goes.

--pt