Add plot-method for `check_dag()` #607

Job	Run time
R-CMD-check / macOS-latest (release)	3m 42s
R-CMD-check / windows-latest (devel)	9m 29s
R-CMD-check / windows-latest (release)	8m 47s
R-CMD-check / ubuntu-latest (devel)	54m 55s
R-CMD-check / ubuntu-latest (release)	5m 33s
	1h 22m 26s

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