Replace length-based tests

[teunbrand]

Hello there,

We have been preparing a new release of ggplot2 and during a reverse dependency check, it became apparent that the prospective ggplot2 3.5.0 would break autohrf.

This PR updates some tests that expected that the result of ggplot() has length 9, which is no longer true in the release candidate. These tests have been replaced with tests that check that the outcome of the plotting functions return a ggplot object.

To test the code changes with the release candidate, you can install it with the code below:

remotes::install_github("tidyverse/ggplot2", ref = remotes::github_pull("5592"))

The release of ggplot2 3.5.0 is scheduled for the 12th of Februari. The progress of the release can be tracked in tidyverse/ggplot2#5588. We hope that this PR might help autohrf get out a fix if necessary.

[demsarjure]

Hi Teun,

This makes sense, thanks for preparing the PR! Just one quick question before I accept it and merge. Is this solution backwards compatible. In other words, will expect_s3_class(plot, "ggplot") work with current version of ggplot and older versions? Thanks.

[teunbrand]

Thank you for the quick response. Yes, the result of ggplot() has been a 'ggplot' S3 class object for many years now I think, so it should be compatible both with the current version of ggplot2 on CRAN and the new release candidate.

[demsarjure]

Looks good and fixes the issue with a new version of ggplot in a backwards compatible manner.