Plot angle between vectors

Question

I am searching for quite a while already now for a function that plots the angle between two arrows / line segments. e.g.:

_{(source: matrix44.net)}

Answer 1

Thanks bgoldst for your effort. I would like to contribute with a rather easy solution that not necessarily reproduces the picture above, but is maybe helpful for less experienced programmers like me:

library(plotrix)

# Creates empty plot and variables
plot(1,type="n",axes=F,xlim=c(-0.1,1.2),ylim=c(-0.1,1.2),xlab="",ylab="")
x1 = c(1,0)
x2 = c(0.4,0.7)

# X1
arrows(0,0,x1[1],x1[2])
text(1,-0.05,expression(x[1]),cex=1.5)

# X2
arrows(0,0,x2[1],x2[2])
text(0.4,0.75,expression(x[2]),cex=1.5)


alpha_angle= acos((x2%*%x1)/(sqrt(x2%*%x2)*sqrt(x1%*%x1)))

draw.arc(0,0,0.15,angle2=alpha_angle)

text(0.07,0.05,expression(alpha),cex=1.5)

In the end this gives the following plot: