Predicting multiple models

In order to see what each of the models for individual categories are doing, it's helpful to make predictions from them. The flow is exactly the same as the flow for making predictions on the whole model, though remember that you only have a single explanatory variable in these models (so expand_grid() isn't needed.)

The models mdl_0_to_15, mdl_15_to_30 and mdl_30_to_45 are available; dplyr is loaded.