library(plotly)

# see https://vimeo.com/202647310
d <- highlight_key(txhousing, ~city, "Select a city")
p <- ggplot(d, aes(date, median, group = city)) + geom_line()
ggplotly(p, tooltip = "city") %>%
  layout(title = "Click on a line to highlight a year") %>%
  highlight(dynamic = TRUE, selectize = TRUE)

# crosstalk keys are automatically added to the group aesthetic...
# if you want to avoid adding the key to group for a layer,
# use the original data
p <- ggplot(d, aes(month, median)) +
  geom_line(aes(group = city)) + 
  geom_smooth(data = txhousing, method = "gam") + 
  facet_wrap(~ year)
ggplotly(p) %>%
  layout(title = "Click on a line to highlight a year")

# perhaps a more useful example
sd <- highlight_key(txhousing, ~year)
p <- ggplot(sd, aes(month, median)) +
  geom_line(aes(group = year)) + 
  geom_smooth(data = txhousing, method = "gam") + 
  facet_wrap(~ city)
ggplotly(p, height = 800, width = 1600) %>%
  layout(title = "Click on a line to highlight a year")