ggplotly and geom_area : display informations when hovering over an area (not a point)

Question

When it comes to a ggplotly graph, it's easy to display informations when hovering over specific point. This code do the job :

toy_df=data.frame("t"=c(seq(1,10),seq(1,10)),
                  "value"=c(runif(10,0,10),2*runif(10,0,10)),
                  "event"=c(rep("A",10),rep("B",10)))

p <- ggplot() + geom_area(aes(y = value, x = t, fill=event), data = toy_df)
ggplotly(p)

But I would like to display informations when hovering over one of the area. Because in my case, area is an event that I want to be able to describe deeply.

Answer 1

Polygons in ggplot2 (geom_polygon) provide a possible solutions.
Below you can find a rather raw code that should clarify the main idea:

library(ggplot2)
library(plotly)
set.seed(1)
toy_df=data.frame("t"=c(seq(1,10),seq(1,10)),
                  "value"=c(runif(10,0,10),2*runif(10,0,10)),
                  "event"=c(rep("A",10),rep("B",10)))

# In order to create polygons like in geom_areas,
# two points on the x-axis must be added: one at t=1 and one at t=10
toy_df2 <- toy_df[NULL,]
for (k in unique(toy_df$event)) {
 subdf <- subset(toy_df, toy_df$event==k)
 nr <- nrow(subdf)
 row1 <- subdf[1,]
 row1$value <- 0
 row2 <- subdf[nr,]
 row2$value <- 0
 toy_df2 <- rbind(toy_df2, row1, subdf, row2)
}
# Stack polygons 
toy_df2$value[toy_df2$event=="A"] <- toy_df2$value[toy_df2$event=="A"] + 
                                     toy_df2$value[toy_df2$event=="B"]

# Calculate mean values for the two events: they will be displayed in the tooltip
toy_df2 <- toy_df2 %>% group_by(event) %>% mutate(mn=round(mean(value),3))

p <- ggplot(data = toy_df2, aes(y = value, x = t, fill=event, 
            text=paste0("Value:", mn,"<br>Event:", event))) + 
     geom_polygon()
ggplotly(p, tooltip="text")