Plot one character on a spatial map

I would like to plot a cex(17) darkgreen triangle at a specific lat/long
spatial map on an existing plot. Or a star at that location.

How might I approach this?

I saw an example (below) but so much like the internet not how to
implement. Besides this is probably for non spatial graphs
http://www.statmethods.net/advgraphs/parameters.html

(you mean pch=17, not cex?)

 uk = raster::shapefile("uk.shp")
 plot(uk)
 points(0, 55, pch=17, col="darkgreen")

works for me. However it depends on what you call a "spatial graph".
You've not given us an example. If you've used `ggplot` instead of
base graphics like I have then `points` won't work...