I am trying to add a line to my bar graph showing the % difference of the years. I was trying to create a function:

year_pct_change <- function(x) {
  x <- x[order(x$Year, decreasing = TRUE), ] # Confirms ordered by decreasing year
  pct_change <- -diff(x$Value)/x$Value[-1] * 100 # Gets percent change in profit from preceding year
  data.frame(year = x$YEAR[-length(x$YEAR)], pct_change = pct_change) # Returns data frame
}

code for the ggplot:

ggplot(Enrolment)    
  geom_bar(aes(x=Year, y=Value),stat="identity", fill="cyan") 
  geom_line(aes(x=Year, y=year_pct_change),stat="identity",color="red",size=2) ,
       x="Year",y="Student Count")

it just throws me an error

Error in f(): ! Aesthetics must be valid data columns. Problematic aesthetic(s): y = year_pct_change. Did you mistype the name of a data column or forget to add after_stat()? Run rlang::last_error() to see where the error occurred.

Any suggestions on how to add a percent difference line to my bar graph? I'm aiming to have something like this..

graph needed

CodePudding user response：

library(dplyr)
library(ggplot2)
    
year_pct_change <- function(x) {
  x[order(x$Year, decreasing = FALSE), ]
  x <- x |> mutate(year_pct_change = 100*((Value - lag(Value))/lag(Value)))
}
Enrolment <- year_pct_change(Enrolment)

ggplot(Enrolment)    
  geom_bar(aes(x=Year, y=Value),stat="identity", fill="cyan")  
  geom_line(aes(x=Year, y=year_pct_change),stat="identity", color="red", size=2)

A couple of changes to the function.

1. Decreasing now FALSE.
2. New formula for % change (to match lengths of arguments and calc % (as pointed out by Rui).
3. Last line of your function unneeded now.

As there is only one y axis, you are unlikely to have the % change and Value with similar ranges.

There was a fragment at the end of your ggplot2 call that I left out: x="Year",y="Student Count"