* R/plot_experiments_matrix.R: Avoid deprecation warning. #175

Job	Run time
Check macOS-latest (release)	3m 50s
Check ubuntu-20.04 (4.0)	45m 24s
Check ubuntu-latest (devel)	18m 35s
Check windows-latest (release)	6m 4s
Check ubuntu-latest (release)	12m 53s
	1h 26m 46s

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