# stack loss gradient
stackloss %>% 
  lm(formula = stack.loss ~ Air.Flow + Water.Temp + Acid.Conc.) %>% 
  coef() %>% 
  as.list() %>% as.data.frame() %>% 
  subset(select = c(Air.Flow, Water.Temp, Acid.Conc.)) ->
  coef_data
# isolines along strongest predictors
scale(stackloss, scale = FALSE) %>% 
  ggplot(aes(x = Water.Temp, y = Air.Flow)) +
  coord_square() +
  geom_point(aes(size = stack.loss)) + scale_size_area() +
  geom_isoline(data = coef_data, text.hjust = 1)
