Error with MatPlotLib subplots

[Danno]

Fixed! Given your great feedback, I thought you deserved a final report...

After much additional effort, I decided that my MacBook installation was corrupt in some way. So I flattened my MacBook - deleted all the contents of the system disk and then:

1. Reinstalled the MacOS,
   
2. Installed only the apps required to test the Python test code.
   

At that point, the test code ran perfectly.

I wish I could find the underlying issue. But what I've learned over the lst 30+ years of coding is that sometimes it's just better to write off an issue as "smelly stuff happens" and move ahead.

Again, many thanks for your help.

Best,

Dan.