need finite 'xlim' values using reactive function in Shiny

Question

I'm trying to build a Linear regression Shiny app with a custom file input. I have a problem with the reactive function in Server.R. The reactive function data returns a data frame called qvdata. When data() is called in renderPlot and I plot from the qvdata I get the following error: "Error in plot.window(...):need finite 'xlim' values". When I do not use the reactive function and do the calculations in renderPlot the error does not occur. Why is this? I want later to be able to call this data reactive function in other render functions to prevent repetition in the code.

Server.R

library(shiny)
shinyServer(function(input, output) {
data<-reactive({
    inFile <- input$file1

    if (is.null(inFile))
        return(NULL)
    #takes in data (In my case .dat file)
    qvdata=read.table(inFile$datapath)
    names(qvdata)=c("H","Q")
    qvdata$Hlog=log(qvdata[,1])
    qvdata$Qlog=log(qvdata[,2])
    xbar=mean(qvdata$Hlog)
    ybar=mean(qvdata$Qlog)
    xdif=qvdata$Hlog-xbar
    ydif=qvdata$Qlog-ybar
    Sxy=sum(xdif*ydif)
    Sxx=sum(xdif*xdif)
    #point estimates of alfa and beta
    qvdata$b=Sxy/Sxx
    qvdata$a=ybar-qvdata$b*xbar
    qvdata
})

output$contents <- renderPlot({
    qvdata=data()
    t=seq(0,2,0.01)
    abline=qvdata$a+qvdata$b*t
    plot(qvdata$Hlog,qvdata$Qlog,type="p",pch=20,col="red")
    lines(t,abline,col="black")
})
})

ui.R

library(shiny)

shinyUI(fluidPage(
    titlePanel('Linear regression'),
    sidebarLayout(
        sidebarPanel(
            fileInput('file1', 'Choose file')   
        ),
        mainPanel(
            plotOutput('contents')

        )
    )
))

Answer 1

Your data function runs the first time when your shiny app starts. Because the input file is missing it will return NULL. But Your plotting function doesn't test its input and is equivalent to plot(NULL) in this case. If you test for an input !is.null() it should work:

output$contents <- renderPlot({
    qvdata=data()
    if (!is.null(qvdata)) {
      t=seq(0,2,0.01)
      abline=qvdata$a+qvdata$b*t
      plot(qvdata$Hlog,qvdata$Qlog,type="p",pch=20,col="red")
      lines(t,abline,col="black")
    }
})