diff --git a/app.R b/app.R
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+++ b/app.R
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+# Credit: build on the example in https://rstudio.github.io/leaflet/shiny.html
+library(sf)
+library(shiny)
+library(spData)
+library(leaflet)
+library(tidyverse)
+world_coffee = left_join(world, coffee_data)
+pal = colorNumeric(palette = "RdYlBu", domain = c(0, 4000))
+
+ui = fluidPage(
+  sidebarPanel(
+    sliderInput("range", "Coffee Production", 0, 4000,
+                value = c(1000, 4000), step = 100),
+    selectInput("year", "Year", c(2016, 2017)),
+    checkboxInput("legend", "Show legend", FALSE)
+  ),
+  mainPanel(
+    leafletOutput("map")
+  )
+)
+
+server = function(input, output, session) {
+  
+  map_centre = st_centroid(world %>% filter(name_long == "Brazil")) %>% 
+    st_coordinates()
+  
+  # This reactive expression returns a character string representing the selected variable
+  yr = reactive({
+    paste0("coffee_production_", input$year)
+  })
+  
+  # Reactive expression for the data subset to what the user selected
+  filteredData = reactive({
+    world_coffee$Production = world_coffee[[yr()]]
+    filter(world_coffee, Production >= input$range[1] &
+                         Production <= input$range[2])
+  })
+  
+  output$map = renderLeaflet({
+    # Things that do not change go here:
+    leaflet() %>% addTiles() %>%
+      setView(lng = map_centre[, "X"], map_centre[, "Y"], zoom = 2)
+  })
+  
+  # Changes to the map performed in an observer
+  observe({
+    proxy = leafletProxy("map", data = filteredData()) %>% 
+      clearShapes()
+    # Show or hide legend
+    proxy %>% clearControls() %>% addPolygons(fillColor = ~pal(Production))
+    if (input$legend) {
+      proxy %>% addLegend(position = "bottomright",
+                          pal = pal, values = ~Production)
+    }
+  })
+}
+
+shinyApp(ui, server)
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