library(ggplot2)
library(Cairo)   # For nicer ggplot2 output when deployed on Linux

ui <- fluidPage(
  fluidRow(
    column(width = 6,
      plotOutput("plot1", height = 350,
        click = "plot1_click",
        brush = brushOpts(
          id = "plot1_brush"
        )
      ),
      actionButton("exclude_toggle", "Toggle points"),
      actionButton("exclude_reset", "Reset")
    )
  )
)

server <- function(input, output) {
  # For storing which rows have been excluded
  vals <- reactiveValues(
    keeprows = rep(TRUE, nrow(mtcars))
  )

  output$plot1 <- renderPlot({
    # Plot the kept and excluded points as two separate data sets
    keep    <- mtcars[ vals$keeprows, , drop = FALSE]
    exclude <- mtcars[!vals$keeprows, , drop = FALSE]
    
    ggplot(keep, aes(wt, mpg)) + geom_point() +
      geom_smooth(method = lm, fullrange = TRUE, color = "black") +
      geom_point(data = exclude, shape = 21, fill = NA, color = "black", alpha = 0.25) +
      coord_cartesian(xlim = c(1.5, 5.5), ylim = c(5,35))
  })

  # Toggle points that are clicked
  observeEvent(input$plot1_click, {
    res <- nearPoints(mtcars, input$plot1_click, allRows = TRUE)

    vals$keeprows <- xor(vals$keeprows, res$selected_)
  })

  # Toggle points that are brushed, when button is clicked
  observeEvent(input$exclude_toggle, {
    res <- brushedPoints(mtcars, input$plot1_brush, allRows = TRUE)

    vals$keeprows <- xor(vals$keeprows, res$selected_)
  })

  # Reset all points
  observeEvent(input$exclude_reset, {
    vals$keeprows <- rep(TRUE, nrow(mtcars))
  })

}

shinyApp(ui, server)