Question

Suppose I draw the following plot composed of two polygons :

library(ggplot2)

polys <- data.frame( 
  id = rep(factor(c("1","2")),each=4),
  x = c(1,2,2,1,2,3,3,2),
  y = c(1,1,2,2,1,1,2,2),
  value = rep(c(3, 4), each=4)
) 

(p <- ggplot(polys, aes(x=x, y=y)) + geom_polygon(aes(fill=value, group=id)))

Which gives :

Is there a way to determine the color given to each polygon from the p ggplot object ? I’ve played a bit with str(p) but didn’t find anything useful for this.

Thanks in advance for any hint !

Answer 1

You can do that by:

> x <- ggplotGrob(p)
> r <- getGrob(x, gPath("geom_polygon", "GRID.polygon"), grep=T, global=T) 
> r
(polygon[GRID.polygon.766], polygon[GRID.polygon.769]) 
> r[[1]]$gp$fill
[1] "#3B4FB8FF" "#3B4FB8FF" "#3B4FB8FF" "#3B4FB8FF"
> r[[2]]$gp$fill
[1] "#B71B1AFF" "#B71B1AFF" "#B71B1AFF" "#B71B1AFF"

1. ggplotGrob makes grob tree
2. getGrob extracts grobs that you want by using gPath
3. $gp$fill shows the color of the polygon.

In the future version of ggplot2, probably you will get information more easily.