What's SHAP contribution dependency plots from xgboost package in R?
On the y axis. That represents the log odd ratios. The formula is log(p/1-p)

P/1-P  P represents probability of success. 1- P represents probability of failure.

When log odd ratio is 0. It means odd is 1. Probability of success is the same as the probability of failure. 

When log odd ratio is below 0, it means that the probability of sucess is less than probability of failure.  

When log odd ratio is above 0, it means that the probability of sucess is higher than probability of failure.