[R] Is it possible to get a downward pointing solid triangle plotting symbol in R?

[Chris Evans]

The reason I am asking is that I would like to mark areas on a plot 
using geom_polygon() and aes(fill = variable) to fill various polygons 
forming the background of a plot with different colours. Then I would 
like to overlay that with points representing direction of change: 
improved, no reliable change, deteriorated. The obvious symbols to use 
for those three directions are an upward arrow, a circle or square and a 
downward pointing arrow.  There is a solid upward point triangle symbol 
in R (ph = 17) and there are both upward and downward pointing open 
triangle symbols (pch 21 and 25) but to fill those with a solid colour 
so they will be visible over the background requires that I use a fill 
aesthetic and that gets me a mess with the legend as I will have used a 
different fill mapping to fill the polygons.  This silly reprex shows 
the issue I think.

library(tidyverse)
tibble(x = 2:9, y = 2:9, c = c(rep("A", 5), rep("B", 3))) -> tmpTibPoints
tibble(x = c(1, 5, 5, 1), y = c(1, 1, 5, 5), a = rep("a", 4)) -> tmpTibArea1
tibble(x = c(5, 10, 10, 5), y = c(1, 1, 5, 5), a = rep("b", 4)) -> 
tmpTibArea2
tibble(x = c(1, 5, 5, 1), y = c(5, 5, 10, 10), a = rep("c", 4)) -> 
tmpTibArea3
tibble(x = c(5, 10, 10, 5), y = c(5, 5, 10, 10), a = rep("d", 4)) -> 
tmpTibArea4
bind_rows(tmpTibArea1,
           tmpTibArea2,
           tmpTibArea3,
           tmpTibArea4) -> tmpTibAreas
ggplot(data = tmpTib,
        aes(x = x, y = y)) +
   geom_polygon(data = tmpTibAreas,
                aes(x = x, y = y, fill = a)) +
   geom_point(data = tmpTibPoints,
              aes(x = x, y = y, fill = c),
              pch = 24,
              size = 6)

Does anyone know a way to create a solid downward pointing symbol?  Or 
another workaround?

TIA,

Chris

-- 
Chris Evans (he/him)
Visiting Professor, UDLA, Quito, Ecuador & Honorary Professor, 
University of Roehampton, London, UK.
Work web site: https://www.psyctc.org/psyctc/
CORE site: http://www.coresystemtrust.org.uk/
Personal site: https://www.psyctc.org/pelerinage2016/