Using unicode characters as shape

Question

I'd like to use unicode characters as the shape of plots in ggplot, but for unknown reason they're not rendering. I did find a similar query here, but I can't make the example there work either.

Any clues as to why?

Note that I don't want to use the unicode character as a "palette", I want each item plotted by geom_point() to be the same shape (color will indicate the relevant variable).

Running

Sys.setenv(LANG = "en_US.UTF-8")

and restarting R does not help. Wrapping the unicode in sprintf() also does not help.

This is an example bit of code that illustrates the problem:

library(tidyverse)
library(ggplot2)
library(Unicode)

p1 = ggplot(mtcars, aes(wt, mpg)) +
  geom_point(shape="\u25D2", colour="red", size=3) +
  geom_point(shape="\u25D3", colour="blue", size=3) + 
  theme_bw()

plot(p1)

And here's what that renders result.

I use macOS Sierra (10.13.6), R version 3.5.1 & Rstudio 1.0.143.

Grateful for any help! I've been scouting several forums looking for a solution and posted to #Rstats, so far nothing has worked. It may be that the solution is hidden in some thread somewhere, but if so I have failed to detect it and I suspect others have also missed it. So, here I am making my first ever post to stack overflow :)