scatter-to-density.R

library(shiny)

mtcars$.key <- row.names(mtcars)

ui <- fluidPage(
  plotlyOutput("scatter"),
  plotlyOutput("bars")
)

server <- function(input, output, session) {
  
  data <- reactive({
    d <- event_data("plotly_selected", source = "scatter")
    mtcars$.color <- ifelse(mtcars$.key %in% d$customdata, "red", "black")
    mtcars
  })
  
  output$scatter <- renderPlotly({
    plot_ly(
      data(), x = ~wt, y = ~mpg, 
      customdata = ~.key, 
      color = ~I(.color),
      source = "scatter"
    ) %>%
      add_markers() %>% 
      layout(dragmode = "lasso", showlegend = FALSE)
  })
  
  output$bars <- renderPlotly({
    ggplot(data(), aes(qsec, fill = .color, alpha = 0.1)) + 
      geom_density() +
      scale_fill_identity() + 
      theme(legend.position = "none")
  })
}

shinyApp(ui, server)