Downloading png from Shiny (R)
A workaround for this strange scenario was discussed on the shiny-discuss google group. What you can do is simply change your reactive plotInput statement into a normal function. Not sure why downloadHandler doesn't play nice with reactive objects.
# change
plotInput <- reactive({...})
# into this
plotInput <- function(){...}
You can also remove the print statement in the downloadHandler call:
output$downloadPlot <- downloadHandler(
filename = "Shinyplot.png",
content = function(file) {
png(file)
plotInput()
dev.off()
})