How to display graphs one below another in the same tab in Shiny Dashboard?

In Shiny Dashboard in a Tab I am trying to plot graphs one below the another, based on the selection of checkbox inputs. Kindly find my UI and Server Code below.

d <-
data.frame(
year = c(1995, 1995, 1995, 1996, 1996, 1996, 1997, 1997, 1997),
Product_Name = c(
  "Table",
  "Chair",
  "Bed",
  "Table",
  "Chair",
  "Bed",
  "Table",
  "Chair",
  "Bed"
),
Product_desc = c("X", "X", "X", "Y", "Y", "Y", "Z", "Z", "Z"),
Cost = c(1, 2, 3, 4, 2, 3, 4, 5, 6)
)

ui <- shinyUI(fluidPage(
useShinydashboard(),
tabPanel(
"Plot",
sidebarLayout(
  sidebarPanel(
  uiOutput('checkbox'),
  #width = 2,
  position = "bottom"),
  mainPanel(uiOutput("graph"))

 )
)
))

My Server code is

server <- function(input, output, session) {
output$checkbox <- renderUI({
checkboxGroupInput("year", "year", choices = (unique(d$year)))
})

output$graph <- renderUI({
# create tabPanel with datatable in it
myTabs = lapply(length(input$year), function(i) {
  tabPanel("Plots",
           fluidRow(plotOutput(paste0("plot", i))))
})

do.call(tabsetPanel, myTabs)
})


observe (lapply(length(input$year), function(i) {
#because expressions are evaluated at app init
#print("I am in Render")
output[[paste0("plot", i)]] <- renderPlot({
  #print ("bbb")
  if (length(input$year) > 0) {
    d %>%
      ggplot(aes(Product_Name, Cost)) +
      geom_col(aes(fill = Product_desc),
               position = position_dodge(preserve = "single")) +
      facet_wrap( ~ input$year[i],
                  scales = "free_x",
                  strip.position = "bottom") +
      theme(strip.placement = "outside") +
      theme_bw()
  }
  })

  }))

  }

When I am using the code, the graph is getting generated, but they are getting overlapped on each other. But I want each graph to be displayed one down to another.

Also the check box options will change dynamically depending upon user selection in some other dashboard pages.

Can someone please suggest me how to overcome this

Thanks in Advance.

David