how to pass ggplot parameters through a function?

Question

I have a simple code that creates a combined barplot and line plot with two axes. I wanted to make this code dynamic so that I can pass the columns to plot as parameters of a function. Here is the code outside the function,it works fine:

data <- data.frame(group = LETTERS[1:5],    # Create example data
                   sample = c(1000, 800, 1200, 900, 800),
                   responses1 = c(0.6, 0.9, 0.9, 0.5, 0.8),
                   responses2 = c(0.2, 0.4, 0.7, 0.1, 0.3)
                   )

ggp1 <- ggplot(data) +                     
  geom_bar(aes(group, sample), stat = "identity") + 
  geom_line(aes(group, responses * max(sample), group = 1),
            col = "#1b98e0", lwd = 3) + 
  scale_y_continuous(sec.axis = sec_axis(~ . / max(data$sample)))

ggp1

Now when trying to put it in a function it becomes challenging a little bit for me:

plot_func <- function(var1, var2) {
  ggp1 <- ggplot(data) +                     
    geom_bar(aes(group, var1), stat = "identity") + 
    geom_line(aes(group, var2 * max(var1), group = 1),
              col = "#1b98e0", lwd = 3) + 
    scale_y_continuous(sec.axis = sec_axis(~ . / max(data[[var1]])))
  
  ggp1
}

If you want to pass them in unquoted you need to quote and unquote using either the older notation !!sym(x) or the newer {{x}}. — Dan Adams
– Dan Adams, Commented Feb 22, 2022 at 19:38
I tried using {{var1}} and {{var2}} but it doesn't work if that is what you mean. — Abdelouahed BEN MHAMED
– Abdelouahed BEN MHAMED, Commented Feb 22, 2022 at 19:43
Can you show the way you define when you call it in plot_func()? It should be plot_func(responses1) — Dan Adams
– Dan Adams, Commented Feb 22, 2022 at 19:45
Your first block of code plotting outside a function also doesn't work because responses is not defined anywhere. — Dan Adams
– Dan Adams, Commented Feb 22, 2022 at 19:50
Ok - will check. But just FYI it's a bad practice to write a function that relies on a specific dataset being defined in your global environment. Much better practice to pass the data to your function so it's self contained. — Dan Adams
– Dan Adams, Commented Feb 22, 2022 at 19:54

langtang · Accepted Answer · 2022-02-22 20:17:18Z

1

If you are passing var1 and var1 unquoted, you could try something like this:

plot_func <- function(df, var1, var2) {
  ggp1 <- ggplot(df) +                     
    geom_bar(aes(group, {{var1}}), stat = "identity") +
    geom_line(aes(group, {{var2}} * max({{var1}}), group = 1), col = "#1b98e0", lwd = 3) + 
    scale_y_continuous(sec.axis = sec_axis(~ . / max(df %>% pull({{var1}}))))
  ggp1
}

Then try:

plot_func(data,sample, responses1)

On the other hand, if var1 and var2 are like this "sample" and "responses1", respectively, you can define the function this way:

plot_func2 <- function(df, var1, var2) {

  var1_max = max(data[[var1]])
  data$line_y = data[[var2]]*var1_max
  
  ggp1 <- ggplot(df) +                     
    geom_bar(aes_string("group", var1), stat = "identity") +
    geom_line(aes_string("group", "line_y", "group" = 1), col = "#1b98e0", lwd = 3) +
    scale_y_continuous(sec.axis = sec_axis(~ . / var1_max))
  ggp1
}

And then use it like this:

var1="sample"
var2="responses1"
plot_func2(data,var1,var2)

edited Feb 22, 2022 at 20:17

answered Feb 22, 2022 at 19:59

langtang

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4 Comments

Abdelouahed BEN MHAMED Over a year ago

Thanks for the working example. I think I wasn't very clear about my problem. I don't know the parameters sample and responses1 to pass them this way: plot_func(data,sample, responses1). I store the chosen two columns selected by the user in two variables var1 and var2 and I wanted to be able to pass them directly like plot_func(data, var1, var2).

langtang Over a year ago

got it. sorry I did not understand

Dan Adams Over a year ago

From your app, are the variables characters? That changes how you want to pull them into your function.

Abdelouahed BEN MHAMED Over a year ago

The user has the list of variables in two dropdown menus, so he selects the var1 and var2 which is are strings (names of columns). After that, I just should plug those two selected values into the function.

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