Not sure if this question is still active but it’s the first one that came up when searching for “saving plots in shiny app” so I wanted to quickly add how to get ggsave to work with downloadHandler along the lines of the original question.

The alternative strategies suggested by juba using direct output instead of ggsave and alternative strategy suggested by alexwhan himself both work great, this is just for those who absolutely want to use ggsave in the downloadHandler).

The problem reported by alexwhan is caused by ggsave trying to match the file extension to the correct graphics device. The temporary file, however, doesn’t have an extension so the matching fails. This can be remedied by specifically setting the device in the ggsave function call, like so in the original code example (for a png):

output$downloadPlot <- downloadHandler(
    filename = function() { paste(input$dataset, '.png', sep='') },
    content = function(file) {
        device <- function(..., width, height) grDevices::png(..., width = width, height = height, res = 300, units = "in")
        ggsave(file, plot = plotInput(), device = device)
    }
)

This call basically takes the device function for a png that ggsave assigns internally (you can look at the ggsave function code to see the syntax for jpg, pdf, etc). Perhaps, ideally, one could specify the file extension (if different from the file name – as is the case here for the temporary file) as a ggsave parameter but this option is currently not available in ggsave.

---

A minimal self-contained working example:

library(shiny)
library(ggplot2)
runApp(list(
  ui = fluidPage(downloadButton('foo')),
  server = function(input, output) {
    plotInput = function() {
      qplot(speed, dist, data = cars)
    }
    output$foo = downloadHandler(
      filename="test.png",
      content = function(file) {
        device <- function(..., width, height) {
          grDevices::png(..., width = width, height = height,
                         res = 300, units = "in")
        }
        ggsave(file, plot = plotInput(), device = device)
      })
  }
))

sessionInfo()
# R version 3.1.1 (2014-07-10)
# Platform: x86_64-pc-linux-gnu (64-bit)
# 
# locale:
#  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
#  [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
#  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
#  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
#  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
# [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
# 
# attached base packages:
# [1] stats     graphics  grDevices utils     datasets  methods   base     
# 
# other attached packages:
# [1] ggplot2_1.0.0 shiny_0.10.1 
# 
# loaded via a namespace (and not attached):
#  [1] bitops_1.0-6     caTools_1.17     colorspace_1.2-4 digest_0.6.4    
#  [5] formatR_1.0      grid_3.1.1       gtable_0.1.2     htmltools_0.2.6 
#  [9] httpuv_1.3.0     labeling_0.2     MASS_7.3-34      munsell_0.4.2   
# [13] plyr_1.8.1       proto_0.3-10     Rcpp_0.11.2      reshape2_1.4    
# [17] RJSONIO_1.3-0    scales_0.2.4     stringr_0.6.2    tools_3.1.1     
# [21] xtable_1.7-3    

Update

As of ggplot2 version 2.0.0, the ggsave function supports character input for the device parameter, that means the temporary file created by the downloadHandler can now be saved with a direct call to ggsave by specifying that the extension to be used should be e.g. "pdf" (rather than passing in a device function). This simplifies the above example to the following

output$downloadPlot <- downloadHandler(
    filename = function() { paste(input$dataset, '.png', sep='') },
    content = function(file) {
        ggsave(file, plot = plotInput(), device = "png")
    }
)