我在获取facet_grid用于评估系统的 ggplot2图时遇到了一些麻烦。绘图呈现良好，但我在浏览器和控制台中收到以下错误：
错误：分面变量必须至少有一个值
每次我根据 input 切换品牌条目时都会发生这种情况input$brand。应用程序不会崩溃，但错误消息很烦人。
我准备了这个可重现的例子：
---
title: "Power ranking for mtcars"
runtime: shiny
output:
flexdashboard::flex_dashboard:
orientation: rows
source_code: embed
---
```{r rows.print = 25}
library(dplyr)
library(ggplot2)
mtcars_tidy <- mtcars %>%
tibble::rownames_to_column() %>%
rename(model = rowname) %>%
mutate(brand = gsub( " .*$", "", model )) %>%
mutate(model = model) %>%
select(brand, model, everything()) %>%
tidyr::gather(key = 'measure', value = "value", mpg:carb) %>%
mutate(ranking = as.factor(sample(x = c(1, 2, 3), size = n(), replace = TRUE))) …