How to define a function and produce its line chart?

See the FAQ: How to do a minimal reproducible example reprex for beginners. Without the skin dataframe I can't be confident in a specific answer.

However, abiline needs more to do its job

abline(a = NULL, b = NULL, h = NULL, v = NULL, reg = NULL,
coef = NULL, untf = FALSE, ...)


a, b are the intercept and slope, single values

and those haven't been provided.

So \dots the next question is how to find \dots