Shiny Download Handler not working for plots

Question

I am trying to save the plots created on my shiny app and it works perfectly fine for the CSV files. However, I am unable to save the plots for ggplot2. What can go wrong in this case?

I get the following error

Saving 7 x 7 in image Warning: Error in Plotnca1: could not find function "Plotnca1"

  output$Plotnca1<-renderPlot({plot(Plot1<-modnca() %>% 
                                      ggplot()+
                                   geom_line(aes_string(y=input$ycol,x=input$xcol,group=(input$subid)))+
                                   theme_bw())})
  
  output$downloadplot <- downloadHandler(
    filename = function() {
      paste("pk_plot", ".png", sep = "")
    },
    content = function(file) {
      ggsave(file, plot = Plotnca1(), dpi = 300,device="png")
    }
  )

Answer 1

I think the primary issue is that output$Plotncal <- renderPlot({...}) doesn't work as we may think. The strategy, I think, is to have your plot as a reactive, call that reactive in the renderPlot call, and in the download handler - at least that's what I've used in the past.

  # Create a reactive object which returns a plot
  plot_ncal_reactive <- shiny::reactive({
    plot <- modnca() %>% 
      ggplot()+
      geom_line(aes_string(y=input$ycol,x=input$xcol,group=(input$subid)))+
      theme_bw())
  
  return(plot)
  })

  # Render the reactive plotting object
  output$Plotnca1 <- shiny::renderPlot(plot_ncal_reactive())

  # Download the reactive object in place of the renderPlot
  output$downloadplot <- downloadHandler(
    filename = function() {
      paste("pk_plot", ".png", sep = "")
    },
    content = function(file) {
      ggsave(file, plot = plot_ncal_reactive(), dpi = 300,device="png")
    }
  )

If the syntax is slightly off, the idea should be correct. If not, a reprex would be really helpful :)

As an aside, it may be worth looking at refactoring from aes_string to aes() and using .data[[input$xcol]], .data[[input$ycol]]

Including an example here that you should be able to copy and paste and run the app to see it works

  library(shiny)
  library(tidyverse)
  ui <- fluidPage(
    shiny::downloadButton(
      outputId = "plotDownload"
    ),
    shiny::plotOutput(
      outputId = "plotOutput"
    )
  )
  
  server <- function(input, output, session) {

    plot_reactive <- shiny::reactive({
      plot <- mtcars %>%
        ggplot(aes(x= mpg)) +
        geom_histogram()
      
      return(plot)
    })
    
    output$plotOutput <- shiny::renderPlot({plot_reactive()})
    
    output$plotDownload <- output$plotDownload <- downloadHandler(
      filename = function() {
        paste("pk_plot", ".png", sep = "")
      },
      content = function(file) {
        ggsave(file, plot = plot_reactive(), dpi = 300,device="png")
      }
    )
  }
  
  shinyApp(ui, server)