(This article was first published on

**The stupidest thing... » R**, and kindly contributed to R-bloggers)I briefly investigated how to draw curved arrows in R. Here’s a small piece of the figure that I ultimately created:

A google search for “curved arrows in R” revealed three options:

`curvedarrow`

in the diagram package- The internal function
`igraph.Arrows`

within the igraph package (mentioned by Gabor Csardi in R help) - Using
`xspline`

for the shaft and adding the head “by hand” (mentioned by Greg Snow)

I wasn’t too happy with how the arrow heads looked using the diagram package, and I didn’t want to have to fiddle around with `xspline`

, and igraph worked quite easily, so I went with that.

Plotting multiple arrows with different colors in a single call to `igraph.Arrows`

didn’t work right (the arrow heads were all the first color), but using a loop and plotting one arrow per call worked fine.

Here’s a bit of code I played with:

library(igraph) par(mar=rep(1,4)) plot(0,0, type="n",xaxt="n", yaxt="n", xlab="", ylab="", xlim=c(0,11), ylim=c(1,11)) x <- 1:10 y <- rep(2, 10) for(k in 0:4) points(x, y+k*2) iArrows <- igraph:::igraph.Arrows mycolors <- rep(c("green", "orange", "red", "blue"), 3) for(i in 1:9) { iArrows(x[i], y[i], x[i+1], y[i+1], h.lwd=2, sh.lwd=2, sh.col=mycolors[i], curve=0.5 - (i %% 2), width=1, size=0.7) iArrows(x[i], y[i]+2, x[i+1], y[i+1]+2, h.lwd=2, sh.lwd=2, sh.col=mycolors[i], curve=1 - (i %% 2), width=1, size=0.7) } for(i in 1:8) { iArrows(x[i], y[i]+4, x[i+2], y[i+2]+4, h.lwd=2, sh.lwd=2, sh.col=mycolors[i], curve=0.5 - (i %% 2), width=1, size=0.7) iArrows(x[i], y[i]+6, x[i+2], y[i+2]+6, h.lwd=2, sh.lwd=2, sh.col=mycolors[i], curve=1 - 2*(i %% 2), width=1, size=0.7) } x1 <- x[1:8] x2 <- x[3:10] y <- y[1:8]+8 iArrows(x1, y, x2, y, h.lwd=2, sh.lwd=2, sh.col=mycolors[1:8], curve=1 - 2*((1:8) %% 2), width=1, size=0.7)

And here’s the corresponding figure:

Note that the arrows at the top all have green arrow heads.

To

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