Plotting numbers with percentages (dual axis?)

Hi - I'm looking to show counts alongside percentages in a graph. In excel, I would probably put the percents on a dual axis as markers with no line. Not sure what the best way to go about this in ggplot2 would be, or if someone can think of a better way to display data in two different formats on the same graph. Here's the code I'm using, thanks for the assistance!


#add carnames as column 
mtcars <- mtcars
mtcars$carnames <- rownames(mtcars)
rownames(mtcars) <- 1:nrow(mtcars)
mtcars$cyl <- as.factor(mtcars$cyl)

#create dataframe for graph
df <-  mtcars %>% 
  group_by(cyl) %>% 
  mutate(unique_cars = n(), 
         num_manual = sum(am), 
         percent_manual = (num_manual / unique_cars)*100) %>% 
  select(cyl, unique_cars, num_manual, percent_manual) %>% 
  distinct() %>% 

ggplot(data = df, aes(x = cyl, y = value, fill = variable))+
  geom_bar(stat = "identity")

You can create a secondary axis using the sec.axis argument in scale_y_ continuous. By defining the total number in your value column, you can create the percentages:

total_value <- sum(df$value)

ggplot(data = df, aes(x = cyl, y = value, fill = variable))+
  geom_bar(stat = "identity") +
  scale_y_continuous(sec.axis = sec_axis(~(./total_value)*100, name = "Percentages"))

Is there a way to refer to the existing y data? Rather than having to pre-calculate the total.