Passing Variable as aes reference

Question

I am building a loop to print a series of plots to separate files in R. Everything works except that when I try to pass a variable to aes in ggplot, the geom_histogram thinks the value is now discrete.

So this works:

epdSimpleName <- c("API", "TI", "CE")

for (epdName in epdSimpleName) {
  plot <- ggplot(simpledf, aes(x=API))
  plot <- plot + geom_histogram(binwidth=5)

  print(plot)
}

but this does not:

epdSimpleName <- c("API", "TI", "CE")

for (epdName in epdSimpleName) {
  plot <- ggplot(simpledf, aes(x=epdName))
  plot <- plot + geom_histogram(binwidth=5)

  print(plot)
}

because R thinks that API, TI, etc is then discrete I guess?

Error: StatBin requires a continuous x variable: the x variable is discrete. Perhaps you want stat="count"?

Thanks for any help/guidance!

dipetkov · Accepted Answer · 2019-04-03 16:56:22Z

1

In this case the error message is confusing and it doesn't give a good hint about what is wrong.

The example that works specifies the aesthetic as aes(x = API). It is important that API is unquoted. The example that doesn't work in effect specifies the aesthetic as aes(x = "API") (not explicitly but that's what happens when you cycle through the loop).

So what it is necessary is to take "API" and "unquote" it. You can achieve that with !!rlang::sym("API")

library("tidyverse")

data(diamonds)

plot <- ggplot(diamonds, aes(x = x))
plot <- plot + geom_histogram()
print(plot)
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

vars <- c("x")

for (var in vars) {
  plot <- ggplot(diamonds, aes(x = !!rlang::sym(var)))
  plot <- plot + geom_histogram()
  print(plot)
}
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

^{Created on 2019-04-03 by the reprex package (v0.2.1)}

answered Apr 3, 2019 at 16:56

dipetkov

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3 Comments

Clive Townsend Over a year ago

Worked perfect, thank you! One follow-up question. How could I have figured that out on my own? I knew that the quotes were the problem but could not find the !!rlang::sym(var) solution. Thanks again! You rock!

dipetkov Over a year ago

This is a very big topic. One place to start learning about tidy evaluation and quosures is the vignette "Programming with dplyr": cran.r-project.org/web/packages/dplyr/vignettes/…. (Even understanding what these terms mean takes some learning.) I also find this blog summary of common transformations useful: edwinth.github.io/blog/dplyr-recipes (You can find this particular transformation under the heading "character to name".)

Clive Townsend Over a year ago

Thanks! I'll take a look!

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