如何使用 rlang 将字符串参数设置为 NULL（用于 facet 变量）？

Question

我正在尝试使用 selectInput 为 ggplot 定义构面，因此 facet_grid 的输入是字符串或列名。其中一个选项是 "none"（不是其中一列）如果未选中，下面的示例只要 "none" 就有效。我正在努力解决如何将 "none" 选项设置为 NULL 的问题。

我尝试了以下操作，但出现以下错误 "Error: cols must be NULL or a vars() specification"。

v <- ifelse(rfacet() == "none", NULL, vars(!!rlang::sym(rfacet())))

# Module - facets options-------------------------------------------------------

   #UI
   plotOpt_rowfacet_UI <- function(id){
      ns <- NS(id)
      uiOutput(ns("ui_opt_rowfacet"))
   }

   #SERVER
   plotOpt_rowfacet <- function(input, output, session, facet_choices="none"){

        output$ui_opt_rowfacet <- renderUI({
         ns <- session$ns

         selectInput(inputId = ns("opt_rowfacet"), label = "Y-variable:",
          choices = facet_choices,
          selected = "none",
          multiple = FALSE, selectize = FALSE)
        })

        return(reactive(input$opt_rowfacet))  #returns column name as string
   }


library(shiny)
   library(dplyr)
   library(tidyverse)

ui <- fluidPage(

  plotOpt_rowfacet_UI("test1"),
  h4("facet selected ouput from module"),
  verbatimTextOutput("y_out"),
  plotOutput("plot")

)

server <- function(input, output, session){

  rfacet <- callModule(module=plotOpt_rowfacet, id="test1", facet_choices= c("none", "mpg", "disp"))

  output$y_out <- renderPrint({
    str(rfacet())
  })

  #this works except if none is selected
  #how can set to NULL if input="none"
  output$plot <- renderPlot({

    #v <- ifelse(rfacet() == "none", NULL, vars(!!rlang::sym(rfacet())))  

    p <- ggplot(mtcars, aes(x=wt, y=disp))+
        geom_point()+
        facet_grid(rows=vars(cyl), cols=vars(!!rlang::sym(rfacet())))
    print(p)
  })


}

shinyApp(ui, server)

Answer 1

你评论的v很接近，你可以做类似下面的事情来定义 output$plot:

output$plot <- renderPlot({
  choice <- rfacet()
  v <- if (choice == "none") NULL else vars(!!rlang::sym(choice))

  ggplot(mtcars, aes(x=wt, y=disp)) +
    geom_point() +
    facet_grid(rows=vars(cyl), cols=v)
})

请注意 renderPlot 文档中的这条评论：

With ggplot2 graphics, the code in renderPlot should return a ggplot object; if instead the code prints the ggplot2 object with something like print(p), then the coordinates for interactive graphics will not be properly scaled to the data space.

如何使用 rlang 将字符串参数设置为 NULL（用于 facet 变量）？

How can you set string argument to NULL with rlang (for use in facet vars)?

r

ggplot2

module

rlang

facet-grid