ggdag warning re: segment.alpha

[tgerke]

This pops up in the whole game exercises, for example

> smk_wt_dag |>
Warning messages:
1: Unknown or uninitialised column: `segment.alpha`. 
2: Unknown or uninitialised column: `segment.alpha`. 
3: Unknown or uninitialised column: `segment.alpha`. 
4: Unknown or uninitialised column: `segment.alpha`. 
+   ggdag(text = FALSE, use_labels = "label")

I searched a bit through {ggdag} but wasn't able to find an obvious source. Not urgent since things work as-is, would just be good to fix to tidy and prevent questions.