map("world", fill = TRUE, col = "gray36", bg = "white", xlim = input$lon_slider, ylim = input$lat_slider)

library(shiny)
library(maps)

ui <- fluidPage(
  sidebarLayout(
    sidebarPanel(
      sliderInput("lon_slider",
                  label = "Longitude",
                  min = -180,
                  max = 180,
                  value = c(-180, 180)),
      sliderInput("lat_slider",
                  label = "Latitude",
                  min = -90,
                  max = 90,
                  value = c(-90, 90))),
    mainPanel(plotOutput("map_plot"),
              plotOutput("map_plot2"))
    )
  )

server <- function(input, output) {

  # Creating map plot
  output$map_plot <- renderPlot({
    req(input$lon_slider)
    req(input$lat_slider)
    map("world", fill = TRUE, col = "gray36", bg = "white", xlim = input$lon_slider, ylim = input$lat_slider)
    title(main = "Zoom by selecting a region on this plot.")
  })

  # Creating map plot
  output$map_plot2 <- renderPlot({
    req(input$lon_slider)
    req(input$lat_slider)
    map("world", fill = TRUE, col = "gray36", bg = "white", xlim = input$lon_slider, ylim = input$lat_slider)
    map("world", fill = TRUE, col = "gray36", bg = "white", xlim = input$lon_slider, ylim = input$lat_slider)
    title(main = "Zoom by selecting a region on this plot.")
  })

}

shinyApp(ui, server)