How to Specify Columns when a user chooses a file to upload in R?

Question

I am writing an R file which prompts a user to upload a file and plots the data in the file the user uploads. I do not know how to reference the columns however (I am trying to use ggplot2) in my code.

The data the user will upload will be a CSV file that would look something like, but can vary:

        January February March April May
Burgers    4       5       3     5    2

I am stuck at the ggplot2 part where I need to reference column names.

server.R

library(shiny)
library(datasets)
library(ggplot2)

X <- read.csv(file.choose())


# Define server logic required to summarize and view the selected dataset
shinyServer(function(input, output) {


  # Generate a summary of the dataset
  output$summary <- renderPrint({
    dataset <- X
    summary(dataset)
  })

  # Show the first "n" observations
  output$view <- renderTable({
    head(X, n = input$obs)
  })

  # create line plot (I took this from https://gist.github.com/pssguy/4171750)
  output$plot <- reactivePlot(function() {
      print(ggplot(X, aes(x=date,y=count,group=name,colour=name))+
              geom_line()+ylab("")+xlab("") +theme_bw() + 
              theme(legend.position="top",legend.title=element_blank(),legend.text = element_text(colour="blue", size = 14, face = "bold")))

  })
})

UI.r

library(shiny)

# Define UI for dataset viewer application
shinyUI(pageWithSidebar(

  # Application title
  headerPanel("Sample Proj"),

  # Sidebar with controls to select a dataset and specify the number
  # of observations to view
  sidebarPanel(
    numericInput("obs", "Number of observations to view:", 10)

  ),

  # Show a summary of the dataset and an HTML table with the requested
  # number of observations
  mainPanel(
    tabsetPanel(
      tabPanel("Table", tableOutput("view")),
      tabPanel("LineGraph", plotOutput("plot"))
    )
  )
))

Answer 1

Here's a working example. I took your code and modified it so that the Column Numbers can be passed from UI.R as inputs. (I use the diamonds dataset in ggplot2 for my dataframe.)

Note that I have created a couple of reactive functions in Server.R.

Server.R

library(shiny)
library(datasets)
library(ggplot2)

#x <- read.csv(file.choose())
x <- diamonds

# Define server logic required to summarize and view the selected dataset
shinyServer(function(input, output) {

  createPlot <- function(df, colx, coly) {
    x <- names(df)[colx] 
    y <- names(df)[coly] 
    ggplot(data=df, aes_string(x = x, y = y) ) + geom_line()
  }

  Y <- reactive({
    x
  })

  # Generate a summary of the dataset
  output$summary <- renderPrint({
    dataset <- x
    summary(dataset)
  })

  # Show the first "n" observations
  output$view <- renderTable({
    head(x, n = input$obs)
  })

  # create line plot (I took this from https://gist.github.com/pssguy/4171750)
  output$plot <- reactivePlot(function() {
    df <- Y()
    print(createPlot(df, colx=input$xa, coly=input$ya))
  })
})

UI.R

library(shiny)

# Define UI for dataset viewer application
shinyUI(pageWithSidebar(

  # Application title
  headerPanel("Sample Proj"),

  # Sidebar with controls to select a dataset and specify the number
  # of observations to view
  sidebarPanel(
    numericInput("obs", "Number of observations to view:", 10)
    ,numericInput("xa", "Column to plot as X-axis:", 5)
    ,numericInput("ya", "Column to plot as Y-axis:", 6)

  ),

  # Show a summary of the dataset and an HTML table with the requested
  # number of observations
  mainPanel(
    tabsetPanel(
      tabPanel("Table", tableOutput("view")),
      tabPanel("LineGraph", plotOutput("plot"))
    )
  )
))

As a separate suggestion, you could first get your shiny app working with a static dataframe, then try the file.choose() option with variable data frames.

Hope this helps you move forward.

Updated based on @joran's comment:

My original response was using the column number inside ggplot's aes with an environment=environment() argument added. I have modified the createPlot function in server.R to use aes_string instead.