404 not found when saving base R plot in Shiny

Question

I'm making a Shiny app that involves making plots from a function from a bioinformatics package that uses base R graphics. I'm building the Shiny app in RStudio Server. Since the plot can only be made with that function in that package, using ggplot2 or highcharter instead is not an option. I have no problem with saving plots made by ggplot2, but got into trouble when trying to save plots made with base R graphics. I used the answer in Downloadhander (save plot) for basic plot in shiny, but when I clicked the download button, I got "404 not found" and download was not initiated, even though that plot displayed properly within the Shiny app. Here's a modified version of RStudio's default Faithful Geyser app that can reproduce this problem:

library(shiny)

# Define UI for application that draws a histogram
ui <- fluidPage(

   # Application title
   titlePanel("Old Faithful Geyser Data"),

   # Sidebar with a slider input for number of bins 
   sidebarLayout(
      sidebarPanel(
         sliderInput("bins",
                     "Number of bins:",
                     min = 1,
                     max = 50,
                     value = 30),
         downloadButton("download", "Download plot")
      ),

      # Show a plot of the generated distribution
      mainPanel(
         plotOutput("distPlot")
      )
   )
)

# Define server logic required to draw a histogram
server <- function(input, output) {
   p <- reactive({
     # generate bins based on input$bins from ui.R
     x    <- faithful[, 2] 
     bins <- seq(min(x), max(x), length.out = input$bins + 1)

     # draw the histogram with the specified number of bins
     hist(x, breaks = bins, col = 'darkgray', border = 'white')
   })
   output$distPlot <- renderPlot(p())
   output$download <- downloadHandler("foo.png", 
                                      content = function(file) {
                                        png(file = file)
                                        p()
                                        dev.off()
                                      })
}

# Run the application 
shinyApp(ui = ui, server = server)

It seems that the problem is related to

png(file = file)
p()
dev.off()

not working within Shiny, since when I tried to save ggplot this way, it also gave "404 not found", while ggsave worked fine to download ggplot (though not base R plots) inside the Shiny app. Outside Shiny, the base R way to save plots works properly.

score 1 · Accepted Answer · answered Oct 19 '18 at 12:47

Changing p() from a reactive to a standard function solved the issue for me.

library(shiny)

# Define UI for application that draws a histogram
ui <- fluidPage(

    # Application title
    titlePanel("Old Faithful Geyser Data"),

    # Sidebar with a slider input for number of bins 
    sidebarLayout(
        sidebarPanel(
            sliderInput("bins",
                        "Number of bins:",
                        min = 1,
                        max = 50,
                        value = 30),
            downloadButton("download", "Download plot")
        ),

        # Show a plot of the generated distribution
        mainPanel(
            plotOutput("distPlot")
        )
    )
)

# Define server logic required to draw a histogram
server <- function(input, output) {
    p <- function() {
        # generate bins based on input$bins from ui.R
        x    <- faithful[, 2] 
        bins <- seq(min(x), max(x), length.out = input$bins + 1)

        # draw the histogram with the specified number of bins
        hist(x, breaks = bins, col = 'darkgray', border = 'white')
    }
    output$distPlot <- renderPlot(p())
    output$download <- downloadHandler(filename = "foo.png", 
                                       content = function(file) {
                                           png(filename = file)
                                           p()
                                           dev.off()
                                       },
                                       contentType = "image/png")
}

# Run the application 
shinyApp(ui = ui, server = server)

404 not found when saving base R plot in Shiny

1 Answers1