R ggplot2: Replacing aes_string() with aes() and .data[[varname]] when varname may be NULL

I am working on updating some older R code to make it use ggplot2's aes() now that aes_string() is deprecated. I admit I don't understand all of the details of tidy evaluation, but from what I do understand it seems like the replacement for aes_string(shape=shapevar) would be aes(shape=.data[[shapevar]]), or possibly aes(shape=!!sym(shapevar)).

The problem with these replacements is that they do not work when shapevar is NULL, as shown in the example below:

library('ggplot2')

set.seed(1)
dat <- data.frame(x=rnorm(25),y=rnorm(25),othervar=factor(sample(1:3,25,replace=T)))

make_scatter_string <- function(plotdat,outfile,xvar='x',yvar='y',colorvar=NULL,shapevar=NULL) {
  plt <- ggplot(plotdat,aes_string(x=xvar,y=yvar,color=colorvar,shape=shapevar)) + geom_point()
  ggsave(outfile,plt)
}

make_scatter_data <- function(plotdat,outfile,xvar='x',yvar='y',colorvar=NULL,shapevar=NULL) {
  plt <- ggplot(plotdat,aes(x=.data[[xvar]],y=.data[[yvar]],color=.data[[colorvar]],shape=.data[[shapevar]])) + geom_point()
  ggsave(outfile,plt)
}

make_scatter_sym <- function(plotdat,outfile,xvar='x',yvar='y',colorvar=NULL,shapevar=NULL) {
  plt <- ggplot(plotdat,aes(x=!!sym(xvar),y=!!sym(yvar),color=!!sym(colorvar),shape=!!sym(shapevar))) + geom_point()
  ggsave(outfile,plt)
}

make_scatter_string(dat,'test1.png',colorvar='othervar') # Works, aes_string can handle shapevar being NULL
make_scatter_data(dat,'test2.png',colorvar='othervar') # Gives error message "Error in `.data[[NULL]]`: ! Must subset the data pronoun with a string, not NULL."
make_scatter_sym(dat,'test3.png',colorvar='othervar') # Gives error message "Error in `sym()`: ! Can't convert NULL to a symbol."

What is the correct way to give aes() a variable like shapevar that may contain the name of a variable in the data or may be NULL? Or is there some other value that should be used to say "don't use this aesthetic" instead of NULL?

I think you can add individual aes(.) directly based on the value of is.null(.).

make_scatter_data <- function(plotdat, xvar='x', yvar='y', colorvar=NULL, shapevar=NULL) {
  plt <- ggplot(plotdat, aes(x=.data[[xvar]],y =.data[[yvar]])) + geom_point()
  if (!is.null(colorvar)) plt <- plt + aes(color=.data[[colorvar]])
  if (!is.null(shapevar)) plt <- plt + aes(shape=.data[[shapevar]])
  plt
}
make_scatter_data(dat, colorvar='othervar')

Similarly,

make_scatter_sym <- function(plotdat, xvar='x', yvar='y', colorvar=NULL, shapevar=NULL) {
  plt <- ggplot(plotdat, aes(x=!!sym(xvar), y=!!sym(yvar))) + geom_point()
  if (!is.null(colorvar)) plt <- plt + aes(color = !!sym(colorvar))
  if (!is.null(shapevar)) plt <- plt + aes(shape = !!sym(shapevar))
  plt
}
make_scatter_sym(dat, shapevar='othervar')

How about:

make_scatter_data <- function(plotdat, outfile, xvar = x, yvar = y,
                              colorvar = NULL, shapevar = NULL) {
  plt <- ggplot(plotdat,
           aes(x={{xvar}}, y={{yvar}}, color={{colorvar}}, shape={{shapevar}})) + 
    geom_point()
  ggsave(outfile,plt)
}


make_scatter_data(dat, "myfile.png")

make_scatter_data(dat, "myfile.png", colorvar = othervar)

make_scatter_data(dat, "myfile.png", shapevar = othervar)

make_scatter_data(dat, "myfile.png", xvar = y, yvar = x, 
                  colorvar = othervar, shapevar = othervar)

Or if you want to feed in quoted names:

make_scatter_data <- function(plotdat, outfile, xvar = x, yvar = y,
                              colorvar = NULL, shapevar = NULL) {
  xvar = sym(xvar)
  yvar = sym(yvar)
  if(!is.null(colorvar)) colorvar = sym(colorvar)
  if(!is.null(shapevar)) shapevar = sym(shapevar)
  
  plt <- ggplot(plotdat,
                aes(x={{xvar}}, y={{yvar}}, color={{colorvar}}, shape={{shapevar}})) + 
    geom_point()
  plt
  # ggsave(outfile,plt)
}

make_scatter_data(dat, "myfile.png", xvar = "y", yvar = "x", 
                  colorvar = "othervar")

Adapting my answer on How to replace the deprecated ggplot2 function aes_string: accepting an arbitrary number of named strings to specify aesthetic mappings?, a flexible and generalized approach of the one by @r2evans may look like so.

Basically I renamed the functions arguments to the names of the aesthetics and used ... to allow to pass additional aesthetics and which also works with NULL:

library(ggplot2)

make_scatter_sym <- function(plotdat, outfile, x = "x", y = "y", ...) {
  args <- lapply(list(x = x, y = y, ...), function(x) if (!is.null(x)) sym(x))

  plt <- ggplot(plotdat, aes(!!!args)) +
    geom_point()

  ggsave(outfile, plt)

  plt
}

make_scatter_sym(dat, "test1.png", color = "othervar")
#> Saving 7 x 5 in image

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make_scatter_sym(dat, "test1.png", color = NULL, shape = NULL)
#> Saving 7 x 5 in image

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