suppressMessages(library(ggthemes))
suppressMessages(library(ggplot2))
suppressMessages(library(dplyr))

cars <- mtcars

data <- 
  cars %>%
  group_by(carb) %>%
  summarise(mpg=mean(mpg))

ggplot(data=data, aes(x=carb, y=mpg)) +
  geom_bar(stat="identity")

data$carb <- factor(data$carb, levels = data$carb[order(-data$mpg)])

ggplot(data=data, aes(x=carb, y=mpg)) +
  geom_bar(stat="identity")

ggplot(data=data, aes(x=carb, y=mpg)) +
  geom_bar(stat="identity") +
  theme_tufte()