# asse del segmento AB
plot(c(-10,10),c(-10,10),type="n",xlab="", ylab="", asp=1)
abline(v=seq(-10,10,1),h=seq(-10,10,1),col="grey")
for(i in 1:2) axis(i,pos=0,label=FALSE,col="brown")
A <- c(-1,3); B <- c(7,-4)
points(A[1],A[2]); points(B[1],B[2]); lines(c(A[1],B[1]),c(A[2],B[2]))
M <- A+(B-A)/2; points(M[1],M[2],col="blue")
if (A[2]==B[2]) abline(v=A[1]+(B[1]-A[1])/2) else
  abline(M[2]+(A[1]-B[1])/(A[2]-B[2])*M[1],-(A[1]-B[1])/(A[2]-B[2]),col="blue")