I fixed my Matplotlib installation. How? Well, not too sure how. But it dawned on me that my system might have more than one Matplotlib configuration file, matplotlibrc, from previous installations in several other places. Indeed, a global search showed that I had three (3) matplotlibrc files. One of them was the "official" one, because it was sitting on the folder where it was expected to be.

The second one was a remnant of an old, now uninstalled, Cygwin-X environment. I uninstalled those Matplotlib leftovers, and a second global search showed that my system was clean of this configuration file and all Cygwin-X Matplotlib residues. But that didn't fix anything. I was still getting the same error messages.

The third matplotlibrc file was created by a wxMaxima/Maxima a CAS package I had recently installed. Curiously, this config file had a size of around 22 KB, which is half the size of a regular matplotlibrc file (44 KB). In this case, I wiped the whole thing out. I uninstalled the CAS completely. That got me rid of the config file. Yet the problem persisted.

Then I installed Texstudio, to see if Matplotlib could somehow use the fonts (?) from its library (?). Again, nothing happened.

Out of despair, I decided to install TeX Live, of which I had never heard of before or had ever installed on my system. After 21 hours (yes, almost a whole day!) of downloads, the installation began and, shortly before its completion, the nicely TeX-decorated charts were back. No system boot required. This made me very happy. I had hundreds of scripts using LaTeX, neat applications that took me days of work to put together - and that all of a sudden became completely useless.

It seems that Matplotlib liked Tex Live. But why was it working before as it always did, when there was no Tex Live at all on my machine? Why did it stop working? I think I will never know.