Linking Download Button in Shiny to a Specific Plot - r

Question

I have a Shiny app which reads in data from a spreadsheet and produces two plots - a box plot and a histogram. I want to be able to download each plot separately (as png) using a download button when I have made various adjustments using sliders etc.
My plots work, and the download buttons work, but will only download one of the plots (apparently the one that was drawn most recently). In a reactive environment this is not very predictable and does not do what I want.
My question is, how do I specify in the download function which of the two plots to download?
An abbreviated version of my code is here:
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
<... various components ...>
}}
h3("Save the Box Plot"),
downloadButton("SaveBox", label = "Save Box Plot as PNG"),
h3("Save the Histogram"),
downloadButton("SaveHist", label = "Save Histogram as PNG")
),
mainPanel(
## Display the Boxplot
plotOutput("BoxPlot"),
## Display the Histogram
plotOutput("Hist"),
)
)
)
server <- function(input, output) {
############
## Read file and process data
graphData <- reactive ({
... draw box plot ...
})
################################
## Display the box plot
output$BoxPlot <- renderPlot({
...
})
################################
## Display the histogram
output$Hist <- renderPlot({
... draw histogram ...
})
################################
## Save to PNG with the plot title as a default file name.
## Skip saving if no file has been loaded
## Save file using the system file save utility so you can name it and put it where you want
output$SaveBox = downloadHandler(
filename = function(file){
ifelse (is.null(input$DataFile),return(), str_c(input$Title, ".png"))
},
content = function(file) {
ggsave(file, width = 290, height = 265, units = "mm", device = "png")
}
)
output$SaveHist = downloadHandler(
filename = function(file){
ifelse (is.null(input$DataFile),return(), str_c(input$HistTitle, ".png"))
},
content = function(file) {
ggsave(file, width = 290, height = 265, units = "mm", device = "png")
}
)
)

Answer 1

The issue is that you have to tell ggsave which plot you want to save. If not specified the last plot will be saved. To fix you code you could move the plot codes into reactives, which could then be called both in the renderPlots and passed to the plot argument of ggsave so that the right plot is saved:
I adjusted your code a bit to make it a reproducible example using mtcars as example data:
library(shiny)
library(ggplot2)
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
h3("Save the Box Plot"),
downloadButton("SaveBox", label = "Save Box Plot as PNG"),
h3("Save the Histogram"),
downloadButton("SaveHist", label = "Save Histogram as PNG")
),
mainPanel(
## Display the Boxplot
plotOutput("BoxPlot"),
## Display the Histogram
plotOutput("Hist")
)
)
)
server <- function(input, output) {
## Read file and process data
graphData <- reactive({
mtcars
})
box_plot <- reactive({
ggplot(graphData(), aes(mpg)) +
geom_boxplot()
})
hist_plot <- reactive({
ggplot(graphData(), aes(mpg)) +
geom_histogram()
})
## Display the box plot
output$BoxPlot <- renderPlot({
box_plot()
})
## Display the histogram
output$Hist <- renderPlot({
hist_plot()
})
output$SaveBox <- downloadHandler(
filename = function(file) {
"box_plot.png"
#ifelse(is.null(input$DataFile), return(), str_c(input$Title, ".png"))
},
content = function(file) {
ggsave(file, plot = box_plot(), width = 290, height = 265, units = "mm", device = "png")
}
)
output$SaveHist <- downloadHandler(
filename = function(file) {
"hist_plot.png"
#ifelse(is.null(input$DataFile), return(), str_c(input$HistTitle, ".png"))
},
content = function(file) {
ggsave(file, , plot = hist_plot(), width = 290, height = 265, units = "mm", device = "png")
}
)
}
shinyApp(ui, server)