R preserve symbol names across indirection - r

Question

I made a plot like so...
ggplot(my_data, aes(x = ttd, y = aval)) +
theme_bw() +
geom_point(alpha = 0.25)
That gave me a nice plot with ttd and aval as my axes labels. I like how it used the names of the arguments as the default labels.
However, I have a bunch of plots like this, and I wanted to abstract it into my own function. But I can't seem to make the plot from inside the function. Here's what I tried:
bw_plot <- function(data, x_, y_) {
ggplot(data, aes(x = substitute(x_), y = substitute(y_))) +
theme_bw() +
geom_point(alpha = 0.25)
}
bw_plot(my_data, ttd, aval)
But I get this error:
Error in (function (..., row.names = NULL, check.rows = FALSE, check.names = TRUE, : object 'x_' not found
I simply want to pass the symbols down from my bw_plot function into ggplot. How can I get it to see my actual column names?
(I also tried passing the column names in as strings and calling as.name on them, but i get the same result.

Answer 1

substitute will correctly "quote" your arguments x_ and y_. However, aes will apply a second round of "quoting" internally, which is what gives you the error. You need to unquote the result of your substitute calls, as you're passing them to aes. This can be done using !! operator from rlang.
library( ggplot2 )
library( rlang )
bw_plot <- function( .data, x_, y_ )
{
xsym <- ensym(x_)
ysym <- ensym(y_)
ggplot( .data, aes(x = !!xsym, y = !!ysym) ) +
theme_bw() +
geom_point(alpha=0.25)
}
Note that the correct function to use is rlang::ensym, rather than substitute, because you are aiming to capture individual symbols (column names). Also, I suggest not naming your argument data to avoid name collisions with a built-in function.
Here's example usage: bw_plot( mtcars, mpg, wt )

Answer 2

The accepted answer works for ggplot2 version 2.2.1.9000 and later. For versions 2.2.1 and prior, it looks like the non-standard evaluation used by aes makes this impossible. Instead, I have to use aes_ which provides an "escape hatch" allowing me to provide the symbols to the function as expected. Here is the solution:
bw_plot <- function(data, x_, y_) {
ggplot(data, aes_(x = substitute(x_), y = substitute(y_))) +
theme_bw() +
geom_point(alpha = 0.25)
}
bw_plot(my_data, ttd, aval)