abline for plot（gam）-add abline to generic drawn smooth term

Question

如何将abline添加到使用通用smoothterms/partials函数创建的GAM (mgcv)的{​​{1}}分类变量的图表中？

脚本示例

plot()

An example of the smoothterm-plot generated through my code

An example of the partial-plot generated through my code