ggplot2: Show difference in values over time with an arrow

Question

I have a data set with scores to questions asked over two years. Each question has a 2015 value and a 2016 value. I would like to plot each and then show the different between the 2015 value and the 2016 value. Did the score go up or down or stay the same? I was thinking it might be useful to connect pairs of points with a line (or an arrow) to show the direction of change, but I'm having a hard time getting ggplot to do this. Here is my code example:

df <- read.table(text = "question y2015 y2016
q1 90 50
q2 80 60
q3 70 90
q4 90 60
q5 30 20", header = TRUE)

g1 <- ggplot(df, aes(x=question))
g1 <- g1 + geom_point(aes(y=y2015, color="y2015"), size=4)
g1 <- g1 + geom_point(aes(y=y2016, color="y2016"), size=4)
g1

Different approaches to visualizing this are welcome.

Answer 1

If you facet by question and put year on the x-axis, you can highlight the trend direction with color and use the x-axis to show the passage of time.

library(reshape2)
library(dplyr)
library(ggthemes)

ggplot(df %>% melt(id.var="question") %>% 
         group_by(question) %>% 
         mutate(Direction=ifelse(diff(value)>0,"Up","Down")), 
       aes(x=gsub("y","",variable), y=value, color=Direction, group=question)) + 
  geom_point(size=2) + 
  geom_path(arrow=arrow(length=unit(0.1,"in")), show.legend=FALSE) +
  facet_grid(. ~ question) +
  theme_tufte() +
  theme(strip.text.x=element_text(size=15)) +
  guides(color=guide_legend(reverse=TRUE)) +
  scale_y_continuous(limits=c(0,100)) +
  labs(x="Year", y="Value")

With this encoding of aesthetics, you probably don't need the legend, and adding arrows to the line segments may be superfluous as well, but I've left them in for illustration.

Answer 2

I think a "dumbbell" chart would work, too. Here I've reshaped your data to long.

df <- read.table(text = "question y2015 y2016
q1 90 50
q2 80 60
q3 70 90
q4 90 60
q5 30 20", header = TRUE)

df.long <- 
  reshape(df, varying = names(df)[2:3],
        direction = 'long',
        #ids = 'question',
        times = 2015:2016,
        v.names = 'perc',
        timevar = 'year'
        )

ggplot(df.long, aes(x = perc, y = question))+
  geom_line(aes(group = question))+
  geom_point(aes(colour = factor(year)), size = 2)+
  theme_bw()+
  scale_color_brewer(palette = 'Set1', name = 'Year')