Phase diagram with vertical lines connecting discrete phases or states

Question

I draw a state diagram using ggplot2 in R. The discrete vertical axis shows several states, and the horizontal axis shows time. Lines connect the start and end points of a phase, with different colours per state. Here is an example:

library("ggplot2")

states <- data.frame(state = factor(c(2, 1, 2, 1)),
                     start = c(0.5, 1.5, 3.5, 5.5),
                     end = c(1.5, 3.5, 5.5, 7.5))

ggplot(states, aes(x = start,
                   xend = end,
                   y = state,
                   yend = state,
                   color = state)) + 
  geom_segment(size = 3)

How can I connect the end point of a phase with the start point of the next phase (in a different state) using a black, vertical line? Each time point has exactly one state, i.e., there are no overlapping states and no empty time points.

Answer 1

One option would be to reshape your data to long and use the group aes to draw the segments as well as the connecting lines via geom_line:

library(ggplot2)
library(dplyr, warn=FALSE)
library(tidyr)

states1 <- states |> 
  mutate(id = row_number()) |> 
  pivot_longer(c(start, end))
  
ggplot(states1, aes(
  x = value,
  y = state,
  color = state,
)) +
  geom_line(aes(group = value),
    color = "black",
    linewidth = 3, lineend = "round"
  ) +
  geom_line(aes(group = id), linewidth = 3, lineend = "round")