library(shiny)
library(ggplot2)

ui <- page_sidebar(
  title = "Example dashboard",
  sidebar = sidebar(
    varSelectInput("var", "Select variable", mtcars)
  ),
  card(
    full_screen = TRUE,
    card_header("My plot"),
    plotOutput("p")
  )
)

server <- function(input, output) {
  output$p <- renderPlot({
    ggplot(mtcars) + geom_histogram(aes(!!input$var))
  })
}

shinyApp(ui, server)