ggvis shiny issue

server.R

library(shiny)
library(ggvis)

shinyServer(function(input, output, session) {  
  
  df <- mtcars
  
  plot_data <- reactive({   
    df$x <- df[,input$x_var]
    # do some dynamic change of the data...
    df <- df[-c(1:runif(1, 1, 10)),]
    df$id <- 1:nrow(df)
    df
  })
  
  selected_plot_data <- reactive({
    df <- plot_data()
    df$selected <- lb$selected()
    df
  })  
    
  lb <- linked_brush(keys = plot_data()$id, "red")
    
  plot_data %>%
    ggvis(~x, ~mpg) %>% 
    layer_points(fill := lb$fill, fill.brush := "red") %>% 
    lb$input() %>% 
    set_options(width=800) %>% 
    bind_shiny('gg', 'gg_ui')
  
  output$table <- renderDataTable({
    df <- selected_plot_data()
    if(any(df$selected)){df <- subset(df, selected)}
    df
  }, options=list(bPaginate=FALSE))
  
})

ui.R

library(shiny)
library(ggvis)

shinyUI(pageWithSidebar(
  headerPanel('mtcars'),
  sidebarPanel(
    tags$head(
      tags$style(type="text/css", "label.radio { display: inline-block; }", ".radio input[type=\"radio\"] { float: none; }"),
      tags$style(type="text/css", "select { max-width: 200px; }"),
      tags$style(type="text/css", "textarea { max-width: 185px; }"),
      tags$style(type="text/css", ".jslider { max-width: 200px; }"),
      tags$style(type='text/css', ".well { max-width: 250px; }"),
      tags$style(type='text/css', ".span4 { max-width: 250px; }")
    ),    
    selectInput(inputId = "x_var",
                label = "Select X Variable to Display",
                choices = c("drat",
                            "wt",
                            "hp"),
                selected = "wt")
  ),
  mainPanel(
    uiOutput('gg_ui'),
    ggvisOutput('gg'),
    dataTableOutput("table")
  )
))