Compare fuel economy of cars

cars.R

library(fueleconomy)
library(dplyr)
library(tidyr)
library(ggplot2)

comp_vehicles <- vehicles %>% 
  filter(cyl == 4, 
         grepl("Utility", class), 
         make %in% c("Honda", "Toyota","Subaru", 
                     "Hyundai", "Kia", "Mazda", 
                     "Mitsubishi", "Nissan"), 
         year > 2007,
         year < 2014, 
         grepl("All-Wheel|4-Wheel", drive)) %>% 
  mutate(transmission = ifelse(grepl("Auto", trans), "Automatic", "Manual")
         ) %>%
  gather(mpg_type, mpg, cty, hwy)

ggplot(comp_vehicles, aes(x = mpg, y = reorder(paste(make, model), mpg, max), 
       colour = transmission)) + 
  facet_grid(mpg_type ~ year, scales = "free_y") + 
  geom_point(aes(size = displ)) + scale_size(range = c(2,4)) + 
  labs(x = "Miles Per Gallon", y = "Make / Model", size = "Engine displacement", 
       colour = "Transmission")