how can I create an arrow or segment with gradient color using R?
Try this,
library(grid)
png("mask.png")
grid.polygon(c(-0.06, 0.06, 0.06, 0.15, 0, -0.15, -0.06),
c(-5, -5, 2.5, 2, 5, 2, 2.5), gp=gpar(fill="black"),
def="native",
vp=viewport(xs=c(-0.15, 0.15), ys=c(-5, 5)))
dev.off()
library(png)
m <- readPNG("mask.png", native=FALSE)
mask <- matrix(rgb(m[,,1],m[,,2],m[,,3]),
nrow=nrow(m))
rmat <- matrix(rgb(colorRamp(c("blue","white","red"))(seq(0,1,length=nrow(m))), maxColorValue=255),
nrow=nrow(m), ncol=ncol(m))
rmat[mask == "#FFFFFF"] <- NA
grid.newpage()
grid.raster(rmat)
Edit: you can reuse it in a plot, e.g.
library(ggplot2)
ggplot(iris) + geom_path(aes(Sepal.Length, Petal.Length, colour = Petal.Width)) +
guides(colour = guide_colourbar()) +
annotation_custom(rasterGrob(rmat, width=unit(1,"npc"), height=unit(1, "npc")),
x = 6, xmax=6.2, y=2.5, ymax=4)