This is the function I wrote to calculate the mean differences which returns the difference data and the final mean difference for a statistic.

MeanDiff.R

#### MEANDIFF FUNCTION ####
getDiff <- function(barPlotData, statistic, roundPlaces){
  
  #use dplyr and tidyr to format the data
  data <- barPlotData %>%
    select(Year=yearID, madePlayoffs, one_of(statistic)) %>%
    spread_(key = "madePlayoffs", value = statistic) %>%
    summarise(Difference=Yes-No)
  
  #rotate dataframe from vertical to horizontal to place above bar plot
  data <- as.data.frame(data %>% spread(key=Year, value = Difference))
  rownames(data) <- "(Playoff - Nonplayoff)"
  
  #find row mean and round to supplied number of decimal points
  m <- apply(data, 1, mean)[[1]]
  data <- round(data, roundPlaces)
  
  return(list(
    data = data,
    mean = m
  ))
}