Please add an API changes note (as this could technically break rcParams that previously relied on breaking at commas). (But given that this makes possible something that was previously impossible, the change is clearly worth it.)
This should also be used for the latex preamble.
See#4371 as well.

attn@pwuertz

In the documentation I used a string list as an example preamble. Didn't this already circumvent the problem matplotlib has/had with comma in strings? Or is this PR trying to enable single-string preambles?

pgf_with_pdflatex = {    "pgf.texsystem": "pdflatex",    "pgf.preamble": [         r"\usepackage[utf8x]{inputenc}",         r"\usepackage[T1]{fontenc}",         r"\usepackage{cmbright}",         ]}mpl.rcParams.update(pgf_with_pdflatex)

In the documentation I used a string list as an example preamble. Didn't this already circumvent the problem matplotlib has/had with comma in strings? Or is this PR trying to enable single-string preambles?

Yes, from within Python scripts. Using the RC file, LaTeX code with commas in it never actually worked.

Oh right, that's correct. +1
Should we update the documentation too or keep it as is for the near future? Just in case ppl might run into that problem when using the new single-string option on a non-up-to-date matplotlib.

You'll need to rebase on master too (https://matplotlib.org/devel/gitwash/development_workflow.html#rebasing-on-trunk).

You'll need to rebase on master too

Let me know if that worked.

The change applied inbackend_pgf.py also needs to be applied intexmanager.py.

Can we pass several usepackage commands in matplotlibrc file? Previously, it was possible by passing comma-separated \usepackage{package1}, \usepackage{package2} to text.latex.preamble option in matplotlibrc file. Now it appears that this is not possible anymore.

I liked the opportunity to create the same-looking graphs using the same matplotlibrc across a project. But now it seems that I need to manually edit every script and include preamble there via rcParams.

Is there still an option to use matplotlibrc for specifying preamble consisting of several lines? I probably just did not catch the idea.

Is there still an option to use matplotlibrc for specifying preamble consisting of several lines? I probably just did not catch the idea.

Just put everything in a single line:
\usepackage{package1} \usepackage{package2}