Using Object in Shiny App that is defined in another code chunk

demo.Rmd

---
title: "Demo"
runtime: shiny
output: html_document
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
df <- head(WorldPhones, 5)
```

```{r eruptions}
shinyApp(

  ui = fluidPage(
    selectInput("region", "Region:",
                choices = colnames(df)),
    plotOutput("phonePlot")
  ),

  server = function(input, output) {
    output$phonePlot = renderPlot({
      barplot(df[,input$region]*1000,
              ylab = "Number of Telephones", xlab = "Year")
    })
  },

  options = list(height = 500)
)
```