app.R

library(shiny)
library(dplyr)

delete_last_row <- function(df){
  df[1:(nrow(df) - 1),]
}

ui <- basicPage(
  plotOutput("plot1", click = "plot_click", dblclick = "plot_dblclick")
)

server <- function(input, output) {
  mycars <- mtcars %>%
    select(wt, mpg)
  
  output$plot1 <- renderPlot({
    if(!is.null(input$plot_click) && input$plot_click$x > max(mtcars$wt)){
      mycars <<- add_row(mycars, wt = input$plot_click$x, 
                         mpg = input$plot_click$y)
    }
    
    if(!is.null(input$plot_dblclick)){
      mycars <<- delete_last_row(mycars)
    }
    
    plot(mycars$wt, mycars$mpg, xlim = c(1.5, 8))
    abline(v = max(mtcars$wt), lty = 2)
    lines(lowess(mycars), col = 2)
  })
}

shinyApp(ui, server)