Search code examples
rggplot2geometryfillggforce

ggplot2 function for filling a circle under a horizonal line?

I am trying to plot a circle in R, then fill a portion above a horizontal line with one color, and a portion below a horizontal line with another color, but I can't quite get it to work.

I've gotten so far as to draw the circle I want, plot a rectangle on top to "erase" the portion of the circle that should be transparent. However, I can't do a similar thing to get the other portion of the circle to layer on top because the white rectangle would cover the first circle.

Ideally, I'd love a way to just define a chord and then remove anything on one side of the chord.

Here's the simple code I have now and an example of what it creates:

ggplot()+
  geom_circle(aes(x0=0,y0=0,r=1,fill="red"),color=NA)+
  geom_rect(aes(xmin=-1,ymin=-0.5,xmax=1,ymax=1),fill="white",color=NA)+
  theme_void()

only bottom half

I was hoping to produce a full circle with the portion seen above in red and the rest of the circle filled in blue.

If it helps, here's the answer for what I want to do in python! How to draw a filled arc in matplotlib But not in R.

Solution

  • Use ggforce::geom_circle() to make the larger portion of the circle, and ggplot2::geom_ribbon() to make the smaller portion. You’ll have to compute the points for geom_ribbon() manually as shown in my example.

    library(dplyr)
library(ggplot2)
library(ggforce)

y_cut <- -.5

c_pts <- tibble(
    deg = 0:360,
    r = 1,
    x = r * cos((deg * pi)/180), 
    y = r * sin((deg * pi)/180)
  ) %>%
  filter(y <= y_cut)


ggplot() +
  geom_circle(
    aes(x0 = 0, y0 = 0, r = 1), 
    fill = "blue", 
    color = NA
  ) +
  geom_ribbon(
    data = c_pts, 
    aes(x, ymin = y, ymax = y_cut), 
    fill = "red", 
    color = NA
  ) +
  theme_void()