How to fill a single 'pch' point on the plot with two-colours?

Question

If you take the code below, how can you change the filling of the third diamond, so it will be half-black, half-white? The solution should apply with any colors.

data <- c(1,2,3)
plot(data, pch=c(23,18,23), cex=c(2.5,3,2.5))

Answer 1

The pch characters are actual font symbols, so you will be limited to what is available in your fonts. Another alternative is to use the primitive plotting commands to make your own "symbols". This is very flexible, although there can be issues with resizing, etc., when mixing the two methods. This method is implemented in the nice my.symbols() example:

require(TeachingDemos)

bwDiamond <- function() {
    plot.new()
    polygon(c(0, 0.5, 1, 0.5, 0), c(0.5, 0, 0.5, 1, 0.5), lty=1)
    polygon(c(0.25, 0.5, 1, 0.75, 0.25), c(0.25, 0, 0.5, 0.75, 0.25), col=1)
}

data <- c(1,2,3)

dev.new(width=4, height=4)
plot(data, type='n')
points(data[1:2], pch=c(23,18), cex=c(2.5,3))
my.symbols(data[3], data[3], symb=bwDiamond, symb.plots=TRUE, inches=0.22)

See also this Q/A: Point symbols in R

Answer 2

@JohnColby : Ok, here is a way to draw a half-coloured circle point, using grid.semicircle from lodplot package. It's not such tidy solution as yours with polygons, but it does the job:

 require(lodplot)
 dev.new(width=4,height=4)
 plot(1:3,type="n")
 grid.semicircle(100,150,10,5)#left circle on plot below
 grid.semicircle(100,150,10,5,side=3,col=1)
 grid.semicircle(200,150,10,5,side=2)#right circle on plot below
 grid.semicircle(200,150,10,5,side=4,col=1)