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

Just a follow-up to pthesling's answer: This one only works if one of the vectors is a multiple of (1,0). A more general approach to plot the angle between any pair of vectors would work as follows:

library(plotrix)
# Creates empty plot and variables
plot(1,type="n",axes=F,xlim=c(-0.1,0.9),ylim=c(-0.1,0.9),xlab="",ylab="")
x0 = c(1,0)
x1 = c(0.6,0.2)
x2 = c(0.4,0.7)

# X1
arrows(0,0,x1[1],x1[2])
text(0.62,0.18,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_angle1= acos((x1%*%x0)/(sqrt(x1%*%x1)*sqrt(x0%*%x0)))
alpha_angle2= acos((x2%*%x0)/(sqrt(x2%*%x2)*sqrt(x0%*%x0)))


draw.arc(0,0,0.15,angle1=alpha_angle1,angle2=alpha_angle2)

text(0.07,0.06,expression(alpha),cex=1.5)

Link to picture