Unit tests and upcoming ggplot2 3.5.0

[teunbrand]

Hi there,

We have been preparing for a new release of ggplot2 and during a reverse dependency check, it became apparent that the prospective ggplot2 3.5.0 would break ggparallel's units tests.

The main culprit is this pattern of unit testing:

https://github.com/heike/ggparallel/blob/4b2ecd96028e519f63073d89a0ba724a7380f4ae/tests/testthat/test-ggparallel.R#L11-L14

We don't recommend saving ggplot objects to disk, as it locks in internal code that may change between versions. While we attempt to keep the user-facing functions somewhat stable, we make no such promises about the internal code. For testing that plots still are the same, we recommend the vdiffr package.

Unfortunately, this issued showed up late on our radar and we plan to release ggplot2 3.5.0 on the 12th of February, so I do realise this is not entirely a timely notice.