Exercise 2.1

df <- bind_rows(mutate(nass.wheat, crop = "Wheat"),
                mutate(nass.corn, crop = "Corn"),
                mutate(nass.soybean, crop = "Soy")) %>% 
  filter(state %in% c("New York", "Michigan", "Indiana", "Montana", "Washington"))

ggplot(df, aes(year, yield)) + 
  geom_point() + 
  geom_smooth() + 
  facet_grid(state ~ crop)
## `geom_smooth()` using method = 'loess' and formula 'y ~ x'