r - Color a designated area?

Question

I am trying to figure out if there is a way to color only half of a circle that I created using grid.circle.

    library(grid)
    grid.circle(x=.5, y=.5, r=.25,gp=gpar(lwd=10))

I want to make the top half blue, and the bottom half leave white.

Thank you for your help!

Answer 1

Using grid.polygon() and some basic trigonometry, you can define a function that'll do this

Some picky care needs to be taken so that the filled semicircle is not distorted when the viewport is non-square. To accomplish this in a way that matches the rules used by grid.circle(), I've set the origin in "npc" units, and the circle radius in "snpc" units. (For more on the meanings of "npc" and "snpc", see ?unit and vignette("grid")):

library(grid)

filledSemiCircle <- function(x_origin, y_origin, radius, fillcolor, top=TRUE) {
    theta <- seq(0, pi, length = 100)
    if(!top) theta <- theta + pi     ## To fill the bottom instead
    x <- unit(x_origin, "npc") + unit(cos(theta) * radius, "snpc")
    y <- unit(y_origin, "npc") + unit(sin(theta) * radius, "snpc")
    grid.polygon(x, y, gp = gpar(fill = fillcolor))
}

filledSemiCircle(0.5, 0.5, 0.25, "dodgerblue")
filledSemiCircle(0.5, 0.5, 0.25, "gold", top=FALSE)
grid.circle(x = .5, y=.5, r=.25,gp=gpar(lwd=10))