Hover tooltip in Shiny using pure ggplot2 solution - r

Question

The goal here is to render a tooltip on hover using a pure ggplot2 solution without any plotly of javascript hacks.
Here is a naive tentative solution (that does not work)
library(tidyverse)
library(shiny)
shinyApp(
ui = fluidPage(
plotOutput("plotCars", hover="hover", width=700,height=300),
verbatimTextOutput("info")),
server = function(input, output) {
hovered <- reactive(nearPoints(mtcars, input$hover, maxpoints = 1) %>%
rownames_to_column("model"))
output$plotCars <- renderPlot({
ggplot(mtcars, aes(x=wt, y=mpg)) +
geom_point() +
geom_point(color="red",data=hovered()) +
geom_label(aes(label=model),data=hovered(),
hjust="inward",vjust="inward",
size=4,color="red",alpha=0.5)+
xlab("Weight(1000 lbs)")+ylab("Miles/gallon")
})
output$info <- renderPrint({
hovered()
})
})
The problem here is that as soon as the plot is re-rendered including the hover information (e.g. a label), the hover event is automatically reset to NULL, thus invalidating the plot.
In practice the above solution almost works, the tooltip for the hovered point is briefly shown, but immediately the input$hover event is invalidated by the new plot and the re-rendering of the plot removed the tooltip since a that point the hover event is now NULL. In fact the tooltip blinks once and then disappears.

Answer 1

The solution is to keep the previous value of the hovered data point avoiding the invalidation. This objective can be achieved using the observeEvent() method and a reactiveVal(). The solution works like this:
the hovered point information is a reactive value (reactiveVal()), initialized with a zero-row tibble with the same columns as the plotted data set.
This initial value allow a smooth visualization in a ggplot2 layer, that would not be possible initializing it to NULL.
the value is updated in response to an hover event, the function observeEvent() by default ignores when an event become NULL (ignoreNULL = TRUE), therefore when the input$hover is invalidated to NULL the value is not updated and remain the same as before
in the plot rendering, the hovered() value is initially a zero-row tibble (but still having the right columns to be compatible with the plot default data) thus not showing anything, later when an hover near a point is performed it will contain the point information.
library(tidyverse)
library(shiny)
shinyApp(
ui = fluidPage(
plotOutput("plotCars", hover="hover", width=700,height=300),
verbatimTextOutput("info")),
server = function(input, output) {
hovered <- reactiveVal(mtcars %>% filter(FALSE) %>% rownames_to_column("model"))
observeEvent(input$hover, {
hovered(nearPoints(mtcars, input$hover, maxpoints = 1) %>%
rownames_to_column("model"))
})
output$plotCars <- renderPlot({
ggplot(mtcars, aes(x=wt, y=mpg)) +
geom_point() +
geom_point(color="red",data=hovered()) +
geom_label(aes(label=model),data=hovered(),
hjust="inward",vjust="inward",
size=4,color="red",alpha=0.5)+
xlab("Weight(1000 lbs)")+ylab("Miles/gallon")
})
output$info <- renderPrint({
hovered()
})
})