@matoxxx I finally got to investigating the problem and found the root of the problem (and an easy solution).
Do you happen to have the adobe-base-14-fonts package installed (or maybe some other Adobe font package)? If so, that is your problem.
I kept being puzzled at why there were not more threads on this subject on the web. As it turns out, this is affecting extremely few of us and this is why:
R, by default, uses a set of fonts, including helvetica (on linux, you can find the list with
pango 1.44 terminated support for bitmap fonts. So any package using pango and bitmap fonts got broken. R does use pango. But the reason nobody is complaining about the problem is because most people don't use bitmap fonts
the adobe-base-14-fonts package however installs (in addition to many
.ttf fonts) a number of bitmap fonts, including one for helvetica. So if you have this package installed (or another one which installs a bitmap version of any of the font that R uses), then things broke for you with the pango update
It is never a good solution to downgrade a package. So you should revert pango to 1.44 and uninstall adobe-base-14-fonts or whatever other package installed bitmap fonts in your machine. There is no doubt that you also have a
.ttf helvetica file (probably from the gsfonts package, but possibly from other very common dependencies) and it will take over once the bitmap fonts are gone.
An alternative solution (nicer but more complex), if you don't want to loose the fonts in that package, is to configure fontconfig so that helvetica (and any other font that R uses) will use the
.ttf file and not the bitmap file (in my case, all the bitmap files were
.pfb files which are Adobe type1 fonts).
If the problem, in your case, is not caused by adobe-base-14-fonts and you don't know what caused it, you can run (on the command line, not in R,
fc-match helvetica. If, instead of returning a
.ttf file it returns some
.pfb or other file, then you can search what installed that file). But I really suspect that adobe-base-14-fonts is the problem package for you as well and that the fix will thus be really easy.
I hope this helped. Thanks to you for pointing me in the right direction by pinpointing the problem to pango 1.44 in the first place.