> print(df)
  person step start end
1    sam    A     0   4
2    sam    B     4   6
3   greg    A     2   7
4   greg    B     7  11

ggplot(df, aes(colour=step)) +
 geom_segment(aes(x=start, xend=end, y=person, yend=person), size=3) +
  xlab("Duration")

step       sam    greg
1    A        0       2
2    B        4       7
3    C        6      11